The woman stepped off Hadda Street into a pair of courier offices in Yemen’s capital. In FedEx and UPS storefronts tucked along shopping centers and travel agencies in San’a, she mailed two Hewlett-Packard printers to the United States.

She used a fake name, address and phone number. She paid in cash. Then she disappeared.

Hidden inside each printer was a bomb powerful enough to down an airplane.

Authorities believe it was the most sophisticated effort yet by al-Qaida in Yemen to strike inside the U.S. Though details are still emerging, a senior U.S. official said evidence points to a plot to blow up cargo planes inside the U.S., either on runways or over American cities.

Alerted to the plot by Saudi intelligence, security officials chased the two packages across five countries, trying frantically over the next two days to prevent an explosion that could have come at any moment.

Several times, the explosive packages were in plain sight. Twice, a bomb was aboard a passenger plane. Once, authorities were just minutes too late to stop a cargo jet with a bomb from departing for its next destination.

The pursuit — recounted to The Associated Press by officials in the U.S., Britain, Yemen, Germany and the United Arab Emirates — shows that even when the world’s counterterrorism systems work, preventing an attack is often a terrifyingly close ordeal.

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For al-Qaida, the two bombs were a significant upgrade over the small device that failed to detonate inside a passenger’s underwear on a U.S.-bound jet last Christmas. This time, the bombers packed four times the explosives.

Instead of relying on a suicide bomber to ignite the fuse, the bombmaker wired these devices to explode using the alarm function of two cell phones. The phones were wired to syringes full of lead azide, a powder that takes only a small electric charge to explode.

The printer cartridges were filled with PETN, an industrial explosive that, when X-rayed, would resemble the cartridge’s ink powder. Used in heavy construction, PETN is stable enough to endure the jostling of a trans-Atlantic flight but extremely volatile if triggered by a small explosion.

Bomb experts say the cell phone alarm probably would have sent an electrical charge into the syringe, heating a filament and igniting the lead azide. That would trigger the PETN.

U.S. counterterrorism officials believe it was the work of al-Qaida’s master bombmaker in Yemen, Ibrahim Hassan al-Asiri, who’s been linked to the Christmas plot.

UPS and FedEx employees screened the packages in Yemen, according to two U.S. officials who, like most people interviewed, spoke on condition of anonymity because they were not authorized to talk to reporters.

In Yemen, cargo screening is done manually, one official said. Employees looked at the contents of the packages but never took the printer apart.

Both packages were cleared for delivery.

It was a breakdown in the first line of defense in the cargo system. The U.S. doesn’t inspect international packages until they arrive, relying instead on shipping companies to do the screening.

The addresses on the packages were outdated locations for two Chicago synagogues. The recipients were figures from the Crusades and the Spanish Inquisition — historic episodes in which Christians persecuted Muslims.

For these reasons, officials believe al-Qaida never intended the bombs to be delivered and hoped instead for an airplane explosion.

The packages were dropped off Wednesday, Oct. 27. The FedEx bomb was loaded aboard a passenger jet, a Qatar Airways plane that seats 144. It left Yemen on Oct. 28, for Doha, Qatar. The UPS bomb left Yemen early that same evening, headed to Cologne, Germany.

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As Thursday evening turned to Friday morning in Riyadh, Saudi Arabia, the CIA station received an urgent call from Saudi intelligence. Two bombs were being shipped from Yemen, bound for the United States. One was UPS, the other FedEx, and the Saudis had both the tracking numbers.

The AP is not reporting some details about the tip at the request of intelligence and administration officials who said doing so would jeopardize national security.

A senior CIA official in Riyadh relayed the tip to the agency’s headquarters in Virginia, where it was early Thursday evening.

CIA officials called the White House, and homeland security adviser John Brennan briefed President Barack Obama, who was in his living quarters.

The FBI called FedEx and UPS, which had participated in a government terrorism drill in August. The exercise: A homemade bomb slipped onto a cargo plane.

U.S. and Saudi authorities put Europe on alert. Britain’s intelligence division, MI-6, also received a tip through its office in Yemen.

U.S. authorities had been monitoring steady intelligence on a possible attack like this since early September, a U.S. official said. In early October, the U.S. received a general tip from the Saudis about a possible al-Qaida effort to down airplanes, intelligence officials said.

Also in late September, authorities intercepted a package from Yemen containing papers, books and other items sent to a Chicago-area Muslim bookstore, a senior U.S. official said. At the time, counterterrorism officials thought perhaps the package included coded messages or was intended to set up contact with allies in Chicago, the official said.

Now, investigators believe al-Qaida just wanted to track the package and see how long it took to get into the U.S. so it could time its bombs more effectively.

The official did not identify the bookstore, but FBI and Internal Revenue Service investigators have recently taken an interest in IQRA International Educational Foundation, a nonprofit Islamic foundation that runs a Chicago-area bookstore.

Financial manager Wahaj Ahmed said this past week that IRS auditors showed up about a month ago to inspect the books. That was around the time the group received a FedEx envelope from a company wanting to do business with IQRA.

The company was based in Yemen, he said.

The FBI arrived a few days ago, asking questions about the envelope.

“They said anything emanating from that area, they were tracking it,” Ahmed said.

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With U.S. intelligence on notice, officials in Saudi Arabia summoned the local liaison for Germany’s Federal Criminal Police into a meeting to discuss the bombs.

When the meeting began, a senior German official said, it 1:34 a.m. Friday in Germany and the UPS bomb was sitting at the airport in Cologne, waiting to leave for England.

The liaison officer hurriedly called Germany and authorities rushed to stop the plane. At 2:40 a.m., police ordered that the package could not leave the country.

It was too late. The cargo plane had taken off 36 minutes earlier.

There had never been a chance to spot the bomb in Germany. UPS is among several “safe” companies, German officials said, so the packages weren’t inspected.

The plane was on its way to central England. On the ground, officials didn’t know for sure whether a bomb was on board, and if so, when it would go off.

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It is a 90-minute flight from Cologne to East Midlands, England.

At the White House, Brennan began calling U.S. intelligence leaders to brief them about the plot.

The FBI called Jewish organizations in the Chicago area, a U.S. official said, and placed two locations under surveillance.

When the UPS plane landed in England, it was just after 10 p.m. Thursday in Washington and 3 a.m. Friday in England. The bombs had begun their journey more than 24 hours before and neither had been found.

British investigators were waiting for plane, tipped off by Saudi, U.S. and German officials. Leicestershire police set up a security perimeter and pulled the package off the plane. Police searched the plane, and even the printer, for hours but found nothing.

Pauline Neville-Jones, British minister of state security, was briefed and Brennan spoke with British Deputy Security Adviser Ollie Robbins. But at 10 a.m. local time, after nearly seven hours of search, police concluded there was no explosive.

The UPS plane was cleared for takeoff to Philadelphia, and on to Chicago.

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While British police were searching the UPS package, the FedEX bomb arrived in Dubai aboard a passenger plane from Qatar, where it had spent the night.

Dubai police, having been tipped off to the package, discovered the bomb shortly after it arrived, according to a UAE official security source. The sun was coming up Friday morning in Washington as investigators in Dubai got the first look at al-Qaida’s deadly device.

The U.S. banned all inbound cargo from Yemen.

At 8:30 a.m. in Washington, the government alerted all cargo carriers: Someone is trying to ship explosives from Yemen into the U.S., and we don’t know how many there are.

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In England, police gave the all-clear. Despite not finding the bomb, authorities cleared the plane for takeoff for Philadelphia, and on to Chicago.

Before it could leave, however, British officials were told about the discovery in Dubai and were urged to look again. Brennan and Robbins spoke by phone a second time, and Dubai officials told British police exactly how to locate the bomb.

At 2 p.m. local time, nearly 12 hours after the UPS bomb arrived in England, police put the security perimeter back in place and resume the search.

Brennan and Robbins talked a third time, then Brennan called Yemeni President Ali Abdullah Saleh, a crucial but sometimes unreliable ally in the U.S.-led effort to wipe out al-Qaida.

At the White House, the plot was a centerpiece of President Barack Obama’s morning security briefing.

Exactly when police in England discovered the bomb remains unclear, but authorities there removed the security perimeter and left the airport at 5:30 p.m. local time.

By then the search was on for all packages coming out of Yemen. FBI and Transportation Security Administration officials boarded cargo planes in Philadelphia and Newark, N.J., on Friday, pulling out packages and searching for bombs.

TSA said the searches were done “out of an abundance of caution.” But at the White House and in capitals around the world, the question was more urgent. Are there more bombs?

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Homeland Security officials alerted Jewish leaders around the country, through what’s known as the Secure Community Network, that synagogues should be on the lookout for suspicious packages from Yemen.

UPS, FedEx and Mideast-based shipper Aramex put a halt to all shipments out of Yemen.

As Obama prepared to address the nation, two U.S. fighter jets escorted Emirates Flight 201 into New York. The flight was from Dubai, and investigators feared packages from Yemen may have been on board.

Obama called it a “credible terrorist threat against our country.” Though he stopped short of blaming al-Qaida in Yemen for the plot, he singled out the group and pledged again to destroy it.

That night, in an unusual move, Brennan released a statement thanking Saudi Arabia for a tip that “helped underscore the imminence of the threat emanating from Yemen.”

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Al-Qaida in Yemen is easily the most tech-savvy of al-Qaida’s affiliate groups. So, intelligence officials monitored jihadist Web sites for days, waiting for some claim of credit.

Finally, a week after the attempted attack, al-Qaida in Yemen sent word late Friday afternoon on a jihadist Web site that it had been behind the plot.

“Our advanced explosives give us the opportunity to detonate them in the air or after they have reached their final target, and they are designed to bypass all detection devices,” the statement said.

Even though the bombs never exploded, al-Qaida declared itself victorious for slipping its bombs past security.

And it pledged there would more bombs, on more planes.