WASHINGTON » The bombing of the Boston Marathon on Monday was the end of more than a decade in which the United States experienced strikingly few terrorist attacks, in part because of the far more aggressive law enforcement tactics that arose after the Sept. 11, 2001, attacks.

In fact, the Sept. 11 attacks were an anomaly in an overall gradual decline in the number of terrorist attacks since the 1970s, according to the Global Terrorism Database, one of the most authoritative sources of terrorism statistics, which is maintained by a consortium of researchers and based at the University of Maryland.

Since 2001, the number of fatalities in terrorist attacks has reached double digits in only one year, 2009, when an Army psychiatrist killed 13 people at Fort Hood, Texas, officials say. That was a sharp contrast with the 1970s, by far the most violent decade since the tracking began in 1970, the database shows. But the toll of injuries in the double bombing in Boston, with three dead and 176 wounded, ranks among the highest casualty counts in recent U.S. history, exceeded only by 9/11, the 1993 World Trade Center attack, the 1995 Oklahoma City bombing and the poisoning of restaurant salad bars with salmonella bacteria by religious cultists in Oregon in 1984.

"I think people are actually surprised when they learn that there’s been a steady decline in terrorist attacks in the U.S. since 1970," said Gary LaFree, a University of Maryland criminologist and the director of the National Consortium for the Study of Terrorism and Responses to Terrorism, which maintains the database.

In the 1970s, about 1,350 attacks were carried out by a long list of radical groups, including extremists of the left and the right, white supremacists, Puerto Rican nationalists and black militants, LaFree said. The numbers fell in the 1980s as the groups were eroded by arrests and defections, and again in the 1990s after the collapse of the Soviet Union, which had inspired or covertly supported some violent leftist groups, LaFree said.

He said there were about 40 percent more attacks in the United States in the decade before 9/11 than in the decade after.

"As a result of 9/11, there’s been a revolution in the way law enforcement treats this problem," LaFree said. "Police agencies led by the FBI are far more proactive. They’re interrupting the plots before the attacker gets out the door."

Spectators at the Boston Marathon described a heavy security presence, as has become standard at public events since 2001, including bomb-sniffing dogs that were deployed before the race. But the attack demonstrated an adage in counterterrorism: Security officials have to be good all the time, and terrorists have to be good only once.

The terrorism consortium counted six past marathons disrupted by violent episodes: three in Northern Ireland and one each in Bahrain, Pakistan and Sri Lanka. The only deaths occurred in Colombo, Sri Lanka, in 2008, when a Tamil Tiger militant blew himself up as a marathon started, killing 14 people and injuring 83 others.

Bruce Hoffman, a terrorism expert at Georgetown University, said that a marathon was a particularly difficult event to secure. "It’s a 26-mile route, densely packed in places, and you can’t search people the way you can for a stadium event," he said.

One other statistic offers a cautionary note as investigators search for clues about the identity of the perpetrators of the Boston attack. About half of the attacks worldwide, and nearly a third of those in the United States, have never been solved, LaFree said.

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