Authorities say one man is responsible for at least half of the bank robberies in Arlington County this year, and the FBI and police hope to figure out who he is before he strikes again.

He has done as many as five robberies since June, authorities say. There have been eight bank robberies in the county this year, compared with three in 2010.

Authorities know what the man looks like from surveillance videos – white, in his 50s, about six feet tall and with a thin build — and they said he appears to coolly execute the crimes. They don’t know much else about him, however.

“If we had enough information to put out who he was, we would,” said Detective Crystal Nosal, an Arlington police spokeswoman.

The man doesn’t cover his face during the robberies, but he wears a cap and glasses. He walks casually into the bank branches he robs, authorities said, and once wore a neon green shirt.

Authorities said that the robber is responsible for four bank robberies from June 6 to Aug. 4 and that he might have committed another Tuesday.

He has targeted bank ­branches on Washington Boulevard and Lee Highway, authorities said, and he has twice robbed a BB&T branch in the 5500 block of Lee Highway — once June 16 and again Aug. 4.

Tuesday’s robbery was at a United Bank in the 5300 block of Lee Highway.

The robberies have not all been done in the same way. In two cases, the robber presented a teller with a note demanding money, authorities said. In the other two, he announced a stickup. He has not shown a weapon, authorities said.

“Sometimes there’s a note. Sometimes he implies he has a weapon. Sometimes he doesn’t,” Nosal said.

Although there were only three bank robberies in Arlington last year, the county has averaged about eight a year in recent years.

Across Northern Virginia — including Arlington, Fairfax, Prince William and Loudoun counties — the number of bank robberies dropped by half from fiscal 2006 to 2010, according to the FBI, whose fiscal year ends Sept. 30. In the FBI’s fiscal 2006, there were 86, compared with 43 in fiscal 2010.

In the District, there were 36 bank robberies in fiscal 2010, compared with 45 in 2006, according to FBI data.

Across the country, bank robberies are also decreasing. There were 5,546 in the 2010 calendar year, down from 6,700 in 2008, data show.

Still, most banks have surveillance cameras in case there is a robbery. Bank tellers are told to give up the cash in their drawers if confronted by a robber, but sometimes a teller will put a dye pack in a bag of money. The pack explodes after the robber leaves the bank and marks the stolen cash with the dye.

BB&T officials declined to discuss security measures. The robber has hit BB&T branches three times, authorities said.

“At BB&T, we are committed to providing a safe and secure environment in which our clients can conduct their banking business,” BB&T spokesman David White said. “We cannot comment on the security measures we employ as to do so would compromise the safety and the security of our clients and employees.”