The bank robberies unfold in a matter of minutes as the masked, highly disciplined assailants threaten employees and customers with assault rifles and handguns and swiftly grab bundles of cash.

Law enforcement officials trying to catch this platoon of bandits credit their organization and planning as much as their speed and firepower for enabling them to carry out six holdups in six months and remain on the run. All have taken place between 10 a.m. and noon in busy areas of the District and Maryland.

The gang probably got its arsenal on the black market, officials said, and is most likely suited with body armor, too. The planning begins well before the crimes, officials said, covering every move.

The robbers have stolen cars and vans for each holdup, sometimes weeks in advance, officials said. Targets appear carefully selected, near major roads that offer several ways to escape. The robbers likely are using lookouts. Their bulky clothing and masks have frustrated attempts to identify them, despite surveillance photos and tapes.

After loading up on cash, the robbers have sped away, disappearing into neighborhoods where they ditch and burn their getaway vehicles. The tactic destroys evidence, officials said, and distracts any officers who might be on their trail.

Since January, the gang has stolen more than $350,000 in holdups in the District and Prince George's County. Law enforcement officials, concerned about the potential for violence, have painstakingly analyzed each crime in hopes of turning up patterns or clues that will bring about arrests.

In interviews, they disclosed new details about the crimes that offer a perspective on the challenges and risks that authorities face in ending them.

Authorities know of no suspects.

"These are not off-the-cuff robberies," said Alex J. Turner, assistant special agent in charge of the criminal division of the FBI's Washington field office.

Although no one has been seriously injured, the robbers have assaulted at least two bank employees during their crimes. They have fired weapons in three holdups, including once at a Prince George's police officer. In their most recent crime, on June 29, the robbers fired two shots into the ceiling of a SunTrust Bank branch in Northwest Washington.

Police have told officers to use extreme caution when approaching reported bank robberies in the area and have urged them to wait for help from tactical response teams before confronting the gunmen.

"They have high-powered weapons and have shown a willingness to use them," said D.C. Police Capt. Michael Reese, whose detectives are investigating the robberies along with the FBI. "Those actions are associated with people who appear to be willing to use any means to make their escape."

Investigators said they think the group has at least three to five members who probably live in the Washington area. Most of the robberies have occurred within a relatively small area of the District and Prince George's County.

Despite the robbers' well-armed appearance, investigators who have studied their actions said they do not believe that most of the group's members have military or tactical training. The strongest influence on the gang might be the 1995 movie "Heat," a drama about a heavily armed band of robbers, or television shows depicting similar crimes, investigators said.

The first robbery generated the biggest take. It occurred Jan. 22 at the Bank of America branch in the 5900 block of Blair Road NW. Three robbers rushed into the bank just after 10 a.m., forcing tellers to hand over about $140,000. An accomplice waited outside in a van.

A fifth man, who was not wearing a mask, told a guard at the rear entrance to go inside the bank, where she was disarmed by the three robbers, investigators said. Police are not sure whether that man was part of the gang.

Officers recovered the van used by the robbers about a half-mile away. It was not set on fire, one of only two not torched by the gunmen during their escapes.

The rest of the robberies occurred with the same precision, although the amount of money taken has diminished considerably. Authorities provided an updated summary of the money taken in each crime last week, and it ranges from the high of roughly $140,000 to a low of about $18,000 in a May 27 holdup in Hyattsville.

Not every crime follows the same exact pattern, investigators said. For example, the robbers stole a surveillance video in the second heist, a March 5 holdup of a Riggs branch in the 7600 block of Georgia Avenue. They did not take surveillance tapes in the other robberies, investigators said.

"They probably figured it wasn't worth the time because they are wearing masks," said D.C. police detective Gail Brown.

The getaway cars and vans have usually been stolen days or weeks before the robberies, from locations in the District and in Maryland. The van used in the first robbery in Washington was stolen about a week earlier in Germantown, investigators said.

In a May robbery in Temple Hills, the gunmen used a minivan that had been reported stolen nearly a month earlier in Northwest Washington, investigators said.

About a week after that bank robbery, investigators said, the gunmen swiped a minivan that they used during their holdup of the Industrial Bank on Rhode Island Avenue NE in mid-June.

The choice of vehicles has also intrigued detectives.

The men have stolen mostly Acuras and minivans, both common on area streets and unlikely to raise much suspicion. The vans offer the gunmen room to maneuver in bulky body armor and with their assault-style rifles, detectives said.

The robbers escaped the June 12 holdup at the Industrial Bank in the 2000 block of Rhode Island Avenue NE in a green minivan, which they set on fire a short distance away before transferring to another van. They also abandoned and torched that van, police have said.

Investigators said they think that one of the gunmen burned his face and arms when he set one of the vans on fire after the Industrial Bank robbery. They urged anyone who might have seen someone with such recent burns to call police.

Banks sometimes attempt to thwart robbers by giving them dye packs that can stain the money. The robbers got such a pack in the SunTrust robbery but appear to have avoided one in another crime.

In all the robberies, witnesses have reported that at least one person remained outside with a getaway van or car while others entered the bank. In at least one robbery, a gunman was overheard speaking into a walkie-talkie or cellular phone, an indication that there were accomplices outside the bank, investigators said.

The bank surveillance tapes and photos show the men in action, hooded and masked, pointing the assault weapons that are strapped on their shoulders and filling containers with cash while employees and customers cower on the floor.

Some of the most vivid images have come from the June 29 robbery of the SunTrust branch. A photographer for WTTG-TV (Channel 5) was nearby on another assignment when the robbers stormed into that bank in the 5000 block of Connecticut Avenue NW.

Police and outside security consultants said that other members of the robbery team might have been deployed in the area, as additional lookouts, beyond the view of the camera. Those additional robbers could pose unseen problems for responding police officers.

"Acting as sleepers, the lookouts could attack the police who confronted the more visible robbers," said Louis R. Mizell Jr., a Bethesda-based security consultant who maintains a database covering thousands of categories of crimes.

Mizell said the robbers might alter their tactics further and strike two banks at the same time -- a change in strategy that seems within the reach of the gang.

"It's not just the bank robberies," Mizell said. "It's the stealing of the vans, the obtaining of the special equipment. Their weapons were probably obtained illegally. Then they still have to hide out. It's not one, it's many operations."

Authorities are offering a reward of up to $50,000 for information leading to the arrest and conviction of the bank robbers. They urge anyone with information to call police at 202-727-9099 or the FBI at 202-278-2000.

The robbers escaped from the SunTrust branch on Connecticut Avenue NW to a stolen van, which they later burned.