The man, identified as William Thomas Duncan, led officers to a 1991 Dodge pickup bearing Idaho license plates, where they said they found an assault rifle, an additional rifle with a scope, an armored vest, a Kevlar helmet and a handgun.
The assault rife, described as an SKS with semiautomatic capacities similar to an AK-47 or M-16, was "in plain view" in the car, Secret Service spokesman Marc Connolly said. Connolly also said officers found 1,000 rounds of ammunition in the truck, as well as several knives and items that could be used in an explosive device.
President Bush was in the White House at the time of the arrest. Officials said he was not in danger.
"As far as the president's concerned, it in no way, shape or form interrupted the president's day," White House press secretary Ari Fleischer said.
Between the truck and the site where Duncan was arrested sits the National Christmas Tree, where, 10 hours later, Bush took part in the annual holiday tree-lighting ceremony. Connolly said that "all indications" were that Duncan's appearance "had nothing to do with the tree lighting."
Authorities have said that White House security has been strengthened--largely in undisclosed ways--since September's terrorist attacks on America. But one visible impact has been a shifting, and often expanded, exclusionary zone around the mansion's grounds.
E Street, which runs just south of the grounds, has been closed to vehicles, but remains open to pedestrians. Two blocks of Pennsylvania Avenue, on the north side, were closed to vehicles after a man fired shots at the mansion and, a few months later, a truck bomb was exploded at the Oklahoma City federal building in 1995.
Periodic efforts by local Washington officials and others to reopen the avenue have been met by strenuous objections from the Secret Service. Each new incident nearby makes it more difficult for those seeking to return traffic to the broad street, which is now a pedestrian mall.
Duncan, believed to be in his mid-20s, had no fixed address. One official said he had been living recently with friends and relatives in Idaho, North Dakota and Arizona.
A spokesman for the U.S. attorney's office, Channing Phillips, said Duncan had not been charged as of late Thursday.
Fleischer said Duncan drew the attention of a member of the Secret Service Uniformed Division because he "acted in a suspicious manner." Connolly said that in patting Duncan down, the agents who patrol the White House grounds and its vicinity discovered the knife.
Duncan was spotted about 7 a.m. in a grassy area near a military monument about 50 yards from the Southwest Gate to the White House. The gate is about 100 yards from the Oval Office.
His pickup was parked two blocks away, on 15th Street, southeast of the White House. The street is a commuter artery and was shut down for about two hours during the morning rush hour while agents from the Secret Service's Technical Security Division examined the vehicle.
Last February, Secret Service officers shot and wounded a man who had fired a handgun outside the south fence.
In 1994, two incidents drew attention to security at the White House: A truck driver, Frank Corder, crashed a small Cessna airplane, in an apparent suicide, into a magnolia tree on White House grounds. Then, a Colorado hotel worker, Francisco Duran, fired several shots at the front of the White House from the sidewalk along Pennsylvania Avenue.