A physician who was behaving suspiciously and had made threatening remarks has been arrested at the Trump International Hotel in Washington after police found an assault-style rifle and handgun in his car, authorities said.
Bryan Moles, of Edinboro, Pennsylvania, was taken into custody at the city centre hotel where he was staying as a guest, police said.
At a news conference, Metropolitan Police Department Chief Peter Newsham said his department and the Secret Service received information from Pennsylvania State Police that a tipster had reported that Moles, 43, was travelling to the hotel armed with weapons and ammunition.
Mr Newsham said Moles checked into the hotel about 30 minutes later. Authorities worked with hotel security to locate his car and Moles himself inside the hotel.
"I was very concerned about this circumstance," Mr Newsham said, "and I believe the officers and our federal partners, and in particular the tipster, averted a potential disaster here in our nation's capital."
A police report said authorities saw a firearm "in plain view" in Moles's vehicle and found another inside the glove compartment. Police seized a Glock 23 pistol, a Bushmaster assault-style rifle and 90 rounds of ammunition.
Hotel spokeswoman Patricia Tang said in a statement that "authorities arrested a guest who was behaving suspiciously", but referred further questions to authorities.
The police chief said Moles is being interviewed, and is co-operating with authorities.
Mr Newsham added that the department does not have enough evidence to charge him with making threats, although a department spokesman earlier indicated that was part of the tip to Pennsylvania authorities.
Moles is an A&E physician at a hospital in Erie, Pennsylvania. He is being charged with carrying a pistol without a licence and having unregistered ammunition.
FBI spokeswoman Minique Crump said the agency initially responded, but referred further questions to the local police, who are taking the lead on the investigation.