The man was stopped around 12:30 p.m. EDT in the Adams Morgan section of Washington. Washington Police Chief Charles Ramsey said the suspect may have threatened President Bush, but he offered no specifics.
There have been no indications of any links to terrorist organizations and officials have said they suspect the man, identified as Jeffrey Cloutier, may have medical problems.
The Secret Service received information Tuesday suggesting a man was driving from Pennsylvania toward the White House with explosives. The Secret Service issued an alert for police from New Hampshire on south to Richmond, Va., to watch for a man traveling with a woman, a small dog and an unspecified type and quantity of explosives.
Washington police stopped a Chevrolet Cavalier registered in Pennsylvania and driven by Cloutier, 33, of Newport, N.H.
Authorities found 10 rifles and six handguns in the car, but Ramsey said no explosives were discovered.
Newport Police Chief David Hoyt said his department was alerted Tuesday night that Cloutier was headed to Washington, and officers notified the Secret Service based on that information.
Hoyt would not discuss what information the informant provided or who it was, but noted it was not a relative.
The Secret Service declined to comment on whether Cloutier had made any threats toward the president, but a New Hampshire law enforcement source said Cloutier had talked of harming Mr. Bush.
"He said President Bush was doing a poor job, and he was going to take care of him and take over," said the source, who spoke on condition of anonymity.
Cloutier's grandmother said in a telephone interview that her grandson had been having problems recently and had sought treatment for epilepsy.
"He needs help and he needs it bad," Marjorie Cloutier said. "I think he was trying to get help and he wasn't getting it."
She said she was not aware, however, of any problems her son might have had with Washington.
Cloutier rented a vehicle from an Enterprise Rent-A-Car office in Claremont, N.H., several days ago, according to sales records from the rental company. He got another car when he arrived in the Philadelphia area, according to Aaron Phinisee, manager of an Enterprise office in Center City Philadelphia.
Cloutier then picked up a white 2001 Chevrolet Cavalier from an Enterprise Rent-A-Car on Route 291 near Philadelphia International Airport Tuesday night, the company said.
Cloutier was held for investigation of possessing unregistered firearms and ammunition. Cloutier's wife was also taken into custody, but a Secret Service source said she is unlikely to face any charges.
CBS News Station KYW-TV reports the tip that led to Cloutier's arrest came from a Mount Laurel, N.J., cab driver who had taken the couple to the car rental office in Philadelphia.
The cab driver, Frank Pauloski, also happens to be an Army veteran and a retired New Jersey state trooper. He told KYW that he immediately thought there was something suspicious about his passengers, but all doubt was removed when they arrived at the airport.
"He told me he was going to see the president because he had had a brain operation and had a chip implanted in his brain," says Pauloski. "He didn't like the way the operation went, and was going to talk to the president about it. He said he was the president's boss, and with the chip implant, he had mental telepathy and could tell what everyone was thinking, which was really out of the ordinary."
Even worse, says Pauloski, when he dropped off the couple, he saw a rifle case wrapped in towels among their luggage.
He says he called the Secret Service immediately and told them what he'd seen and heard.