A gang of robbers brandishing military-style assault weapons held up a Prince George's County bank yesterday, marking the fourth time this year that the group has struck in the area, authorities said.

The robbers escaped with about $18,000, bringing their take this year to more than $300,000, authorities said. In all four robberies, the bandits were well armed, wore masks and acted with extreme precision, they said.

It has been years since a gang of bank robbers has been so heavily armed and has raised as much concern among area law enforcement officials, authorities said. No one has been seriously hurt, though the robbers yesterday assaulted a bank employee, who suffered minor neck bruises, they said.

No gunshots were fired yesterday, authorities said. Concern about the gang was heightened two weeks ago when the bandits opened fire for the first time, just missing a Prince George's officer who had rushed to the scene of the robbery, they said.

The gang has struck twice in the District and twice in Prince George's, authorities said.

The heavy firepower, rarely seen in area bank robberies, has law enforcement officials warning about the possibility that a bank employee, customer or police officer could be seriously hurt.

D.C. police Capt. Michael Reese, whose department is working with the FBI and Prince George's police on the cases, said the gang has displayed more sophistication than typical bank robbers. Authorities said the holdups also are netting far more cash than most bank robberies -- an average of about $75,000 a heist, compared with the national average of $5,000.

"They were synchronized," Reese said of the robbers. "There was some level of planning. They probably practiced together . . . . These are pretty serious guys."

Yesterday's episode unfolded shortly before noon at the Chevy Chase Bank branch in the 5800 block of Riggs Road in Chillum. Three masked men in dark trench coats walked into the bank carrying assault weapons and ordered everyone to get on the floor, police said. The robbers went behind the counter and filled up bags with cash, police said.

Witnesses said that the robbers yelled, "Don't move, don't panic, no one will get hurt." The bandits demanded that tellers open the safe, but the employees insisted they did not have access, the witnesses said.

After seven minutes inside the bank, the robbers fled in a minivan, which was later found torched in the 600 block of Kansas Avenue in Takoma Park. Authorities said they found a second torched vehicle, an Acura Legend, in the 3100 block of Elm Street NE. Authorities said they believe the Acura also played a part in the crime.

People inside the bank during the robbery included a 78-year-old woman who said that the robbers were "just a bunch of no-good people."

The tactics yesterday fit the pattern of the earlier crimes, authorities said. In two earlier robberies, the bandits ditched getaway cars and torched them, they said. In another instance, they abandoned a getaway car but did not set it afire, they said.

The first of the robberies was the morning of Jan. 22, when five men stormed into a Bank of America branch at 10:10 a.m. in the 5900 block of Blair Road NW. After quickly subduing an armed guard, the men barked out profanity-laced orders, telling people to lie on the floor and demanding cash, said Detective Gail Brown, a bank robbery investigator with the D.C. police.

Within six minutes, the men exited the bank, jumped into a green Dodge Caravan and sped off, police said. A mile from the bank, the men abandoned the Dodge, which had been stolen from Montgomery County, and jumped into a silver BMW SUV, police said. They stole $140,000 in that robbery, police said.

The robbers next struck March 5, police said. At 10:15 a.m., three men entered a Riggs Bank branch in the 7600 block of Georgia Avenue NW. A fourth man was stationed in a getaway car and another acted as a lookout, police said.

That robbery took less than six minutes and about $94,000 was stolen, authorities said. The robbers fled in a black Acura that was found, abandoned and torched, in a block of Laurel Street NW.

The third robbery occurred shortly before noon May 10, when three men entered a Chevy Chase Bank branch in the 3600 block of Silver Hill Road in Suitland, police said. They stole about $50,000, police said.

A Prince George's police officer arrived as the robbers were emerging from the bank. While the officer was at her cruiser, the bandits opened fire, police said. Two gunshots hit the police car, barely missing the officer, police said. She did not return fire, they said.

The robbers fled in a van, which was found torched at Frankford and 33rd streets in Southeast Washington, police said.

Authorities have been combing records of recently released convicted bank robbers and following other leads. They urge anyone with information to call the FBI at 202-278-2000 or D.C. police at 202-727-9099.

Staff writer Jamie Stockwell contributed to this report.

Surveillance photos show robbers stealing about $50,000 on May 10 from a Chevy Chase Bank branch in Suitland.