The FBI is joining the Honolulu Police Department’s search for the pistol-wielding "backpack bandit" believed responsible for at least four bank robberies on Oahu this year.

The FBI announced yesterday it is offering a $10,000 reward for information leading to his arrest. That is in addition to a $1,000 CrimeStoppers reward.

Authorities have dubbed the robber "the backpack bandit" because he has pulled a gun from a backpack in each of the robberies.

The robber "appears to be escalating his level of violence and we are concerned that in the future someone might get hurt," said Maj. Richard Robinson, head of HPD’s Criminal Investigation Division.

Bank robberies in Hawaii typically consist of a robber showing a note to a teller demanding money, Robinson said. "In this case, he’s brandishing a firearm, going up to multiple tellers. It’s a level of violence we’re not accustomed to here in Hawaii."

FBI special agent Tom Simon said that in one case, a pregnant teller was taken to a hospital for stress symptoms after encountering the robber.

Police cannot be 100 percent certain the same person is involved in all four robberies, but "based on certain indicators, we believe that it is," Robinson said.

The robberies attributed to the backpack bandit occurred:

» March 31 in Kalihi, at the Hawaii Federal Credit Union at 1244 Kaumualii St.

» May 26 in Kalihi, at American Savings Bank at 1851 N. King St.

» Sept. 1 and Nov. 26 in Kunia, at Bank of Hawaii at 94-661 Kupuohi St.

In each of the robberies, the FBI said, the robber approached a bank teller and placed a dark-colored backpack on the counter. He pulled a black semiautomatic handgun from the backpack, pointed the gun at the teller and verbally demanded money.

Robinson said money was taken in each robbery, but he would not say how much. Police do not know if there was a vehicle involved in any of the cases and it appears the man is working alone, the major said.

The backpack bandit is described as a man in his 30s, 5 feet 5 and 190 pounds. In three of the robberies, the robber wore sunglasses resting on top of a dark-colored hat. During three of the robberies, he also appeared to wear a wig.

The FBI is assisting police with the case because HPD’s leads appear to have dried up. "It’s unusual to have bank robberies of this type, with (surveillance camera) images of this quality where we don’t have as many tips from the public," Robinson said.

Simon said "serial bank robbers are extremely rare in Hawaii."

There have been 23 bank robberies on Oahu this year. "Eight of these remain unsolved — four of these we attribute to the ‘backpack bandit,’" Simon said. "To put those numbers in perspective, there were 16 bank robberies on Oahu in 2009 and all of them have been solved."

Police last week arrested a suspect in connection with the Kunia robberies. But after interviewing the man, he is no longer a suspect in any of the robberies, Robinson said.

Bank robbery suspects typically face prosecution by U.S. attorneys in the federal courts. But since 9/11 and the shift in focus to terrorism and counterterrorism efforts, the FBI has generally left investigations into robberies in the hands of local authorities.