A North Carolina tobacco farmer yesterday drove a tractor that he claimed was carrying explosives into a pond on the Mall, leading to a marathon standoff with law enforcement agents that created massive traffic tie-ups and sparked security fears among already tense federal and local officials and residents.

The man, identified by law enforcement sources as Dwight W. Watson, 50, of Whitakers, N.C., drove into the pond in Constitution Gardens, between the Washington Monument and Lincoln Memorial, about noon. As of early this morning, law enforcement officers continued to keep watch on the man, whom they described as distraught. Although they had made contact with him by cell phone, it remained unclear why he was there and what he wanted.

Sgt. Scott Fear, a spokesman for the U.S. Park Police, said the man arrived at the area, just south of Constitution Avenue, in a jeep, towing the tractor on a flatbed trailer. He jumped the curb and drove the jeep into the nearby pond. Then, Fear said, he mounted the tractor and drove it off the trailer bed and into the shallow water.

The man, who began playing music, wore a T-shirt, bearing a variety of badges or patches, and a helmet with what appeared to be a red cross emblem.

Painted on the green John Deere tractor were the messages "Salute to Veterans" and "God Bless the Troops." An American flag flew from the vehicle, mounted upside down in the traditional signal of distress. Another flag depicted tobacco leaves.

Acquaintances said the man had once served in the military, but it was unclear whether yesterday's actions were linked to his service, the possible war with Iraq or concerns about U.S. tobacco policies.

About 100 law enforcement officers -- from agencies including the U.S. Park Police, the D.C. police and the FBI -- cleared the immediate area and closed nearby streets, including Constitution Avenue between 15th and 23rd streets NW, and began efforts to talk to the man.

"We're trying to figure out what he wants and what he needs," Fear said.

Authorities said Watson claimed to have ammonium nitrate, which can be used in fertilizer and explosives. Timothy J. McVeigh used a massive bomb made of ammonium nitrate and fuel oil in the 1995 attack in Oklahoma City that killed 168 people.

The road closings remained in effect through the evening, creating gridlock downtown during the afternoon rush hour.

Acquaintances contacted in North Carolina described Watson as a good-hearted, intelligent and educated man who has strong convictions. As far as they knew, he had never been in trouble, they said.

George Watson of Clemmons, N.C., who said he is Watson's brother, hadn't heard of yesterday's incident until he was reached by a reporter last night. He described his brother as "a fine fellow'' who had served in the Army's 82nd Airborne Division.

Last month at the annual meeting of the Tobacco Growers Association of North Carolina, Dwight Watson and other farmers complained that government controls have made the price of U.S. tobacco so high that it accounts for only 5 percent of the leaf used around the world.

"Why is the American tobacco farmer being put out of business when we grow the best tobacco in the world?" Watson asked, according to an Associated Press account.

Beth Taylor, a neighbor of Watson's, said that if Watson's actions were connected to his concern with government tobacco policies, he probably intended no harm but was "just trying to make a point."

Watson "has just been pushed to the edge," she added. "I wouldn't say he's a crazy person."

J. Claude Mayo, chairman of the county commissioners of Nash County, where Watson lives, said he found it difficult to believe that Watson's actions yesterday were prompted by concerns about the tobacco program.

"He must be experiencing some problems of which we are not aware," Mayo said.

Yesterday's trouble, coming as the nation prepared for a possible war, prompted discussion about whether security should be tightened further, particularly near federal buildings and monuments.

"We're evaluating the situation to see what else could be done to prevent this from happening again," Fear said.

Concrete barriers have been placed in front of many federal buildings, but not near Constitution Gardens, an open park area with few barriers. Local officials said that they expect residents and tourists to continue viewing public museums and monuments.

"Federal law enforcement should determine the appropriate level of security," said Tony Bullock, a spokesman for D.C. Mayor Anthony A. Williams (D). "But we do not want to see a physical barrier or a moat or a medieval construct" blocking the Mall and monuments.

Del. Eleanor Holmes Norton (D-D.C.) issued a statement yesterday morning calling on federal officials to "do whatever is necessary to maintain normal life and operations in the nation's capital," saying fear of war and terrorism have led to some ineffective security remedies.

Asked later whether the standoff changed her views, Norton said officials cannot stop all threats.

"Incidents like this bring out nuts. We must be prepared for a lot of that, too, but nothing approaching the size of a tractor could have gotten close to federal buildings," she said.

The area near the pond is home to several federal agencies, including the Department of the Interior, but no buildings were evacuated.

At the Federal Reserve, officials told anyone working near windows facing the Mall to move deeper inside the building, said Dave Skidmore, a spokesman for the agency. Nonessential employees were allowed to leave at 4:30 p.m.

Staff writers Petula Dvorak, Christopher Lee, Martin Weil and Debbi Wilgoren and researcher Bobbye Pratt contributed to this report.

A man identified as Dwight W. Watson gestures from the flag-bedecked tractor, which he drove into a pond.A police officer directs traffic at 17th Street and Pennsylvania Avenue NW during the evening rush hour. Streets in the area remained closed last night.