A man drove a tractor into a pond Monday between the Washington Monument and the Vietnam Veterans Memorial, police said.
Authorities closed streets in the area, several blocks from the White House. They said they were communicating with the man, who was wearing military fatigues and a helmet with a red cross on it.
Police declined to make the man's identity or motivation public, but the Washington Post quoted unidentified law enforcement sources as saying he was Dwight W. Watson, 50, a tobacco farmer from Whitakers, N.C., and claimed to be carrying explosives.
The U.S. Park Police; FBI; Bureau of Alcohol, Tobacco, Firearms and Explosives; and Secret Service joined city police and blocked off much of the area around the Mall.
The standoff continued past midnight, but authorities said they were in no hurry to resolve it.
"We've got a nice, safe perimeter set up and we're going to be patient with him," said Sgt. Scott Fear of the U.S. Park Police.
The tractor was in a pond near the Reflecting Pool, in the area known as Constitution Gardens. The tractor had been pulling a military-style jeep and a trailer, and the jeep was partially submerged. The tractor had messages on its cab supporting veterans.
The man was sitting in the cab of the tractor, which was in about 3 feet of water. He was playing recordings of military exercise cadences.
The episode started about noon and had traffic snarled long after dark as police had to close off streets in the area.