At least three armed men held up two Brinks armored car guards in a daylight robbery at Dulles International Airport early yesterday, taking two bags apparently containing cash and other documents valued at more than $100,000, officials said.

The robbery occurred about 6:40 a.m. at a Federal Express cargo bay in the northwestern end of the airport complex. According to the FBI and the Metropolitan Washington Airports Authority, which operates Dulles and National airports, three to five men disarmed and injured two Brinks guards while a third Brinks guard sat in the truck, apparently unaware of the robbery.

Officials said the assailants escaped in a late-model Chevrolet van. The two guards were treated at Reston Hospital Center and released. The FBI, which is leading the investigation, termed the robbers "armed and dangerous."

FBI spokesman Jim Mull declined to say specifically how much or what was stolen. He said the canvas bags may have contained a combination of cash, non-negotiable financial documents and other items, possibly including jewelry. "It was a sizable haul, a large amount," said Mull, adding that $100,000 was a "very conservative" estimate of the value of the contents.

No previous robbery of a Brinks armored car has been reported at either Dulles or National, according to William A. Halligan, airport authority public safety chief. Marven Moss, a spokesman for the armored car firm in Darien, Conn., declined to comment.

The cargo area of Dulles, near several office buildings, is a 24-hour operation in an area open to the public. Officials said more than 25 million pounds of freight passed through the airport in March.

The holdup occurred on the street side of cargo bay 90, according to airport authority spokesman Dave Hess. Yesterday afternoon, cargo bays 83 through 90, which are connected, were roped off by investigators. A Brinks armored car was backed up to two of the bays, its rear and side doors open, while FBI agents and airport authority police combed it for evidence.

Dulles officials said each shipper arranges for its own security services, although the airport police force is available to supplement private efforts upon request. Halligan said the airport force had not been contacted in advance about yesterday's shipment.

Officials said they could not reveal the source and intended destination of yesterday's shipment. Federal Express spokesman Armand Schneider in Memphis would not comment on the robbery. The company leases cargo bay 90 and has its own fleet of cargo jets. Federal Express handled more than 7 million pounds of freight at Dulles in March, not counting mail, according to the airport authority.

A Reston Hospital Center spokeswoman identified the injured Brinks guards as Kenneth Hershey, 55, and Cedrick Shelton, 22. She said Hershey was treated for a head injury and Shelton was treated for chest and neck cuts.

Officials said the robbery occurred after the armored car backed up to the Federal Express loading dock.

Two guards apparently entered the building and returned to the truck with two bags.

At least two of the assailants brandished weapons and took the weapons of two guards.

The robbers then attacked the guards, grabbed the bags and fled, officials said.

"The driver was apparently un- aware until the guards came around," said Hess, the airport authority spokesman. He said the robbery was performed quickly.

The FBI described the getaway vehicle as a 1987 or 1988 Chevrolet custom van with a raised roof. The van is described as white with a blue line around its sides and having a blue canvas-covered tire on the back. The van may have Maryland license plates, the FBI said.

Local and state police officials said they had not been brought in on the case, though they said they had been asked to be on the lookout for several men, two of whom were carrying firearms.

People with information about the robbery are asked to call the Washington area FBI field office at (202) 252-7801.

Brinks Inc., founded in 1859, operates in 45 countries. It has about 4,600 employees and operates about 1,000 armored vehicles in the United States, according to a spokesman.

Staff writers Alison Howard, Nell Henderson and Maria Koklanaris contributed to this report.