A chunk of brick or stone struck the roof of Vice President George Bush's limousine as he traveled to work yesterday morning, setting off a massive law enforcement investigation that concluded "there was no assault on the vice president."

But before that conclusion was reached, about 150 law enforcement officers conducted door-to-door searches, interviewed scores of bystanders and formed a human chain to sweep L Street NW after Bush had reported hearing a "loud bang" near 21st and L streets NW and Secret Service agents found a half-inch tear in the vinyl cover of the roof of his armored Cadillac.

District of Columbia police, FBI and Secret Service agents sealed off L Street between 20th and 23rd streets NW for three hours while police helicopters whirred overhead, Special Operations Division teams and police with flak jackets and dogs went from building to building, and officials sweeping the road collected bits of nails and rocks in clear plastic bags in an attempt to determine if an assailant was involved.

In the end an FBI laboratory analysis of fragments imbedded in the roof of Bush's automobile concluded that the object that struck the car contained clay and cement and was "consistent with building materials in the area." Several buildings are under construction in the area, including a luxury condominium project at the corner of 22nd and L streets.

Closing part of L Street NW, a major artery into downtown, created a rush-hour traffic jam that delayed motorists up to an hour, according to D.C. police. The area did not clear of traffic congestion until about 10:30 a.m., when the barricades were lifted.

By midday, when he appeared on Capitol Hill for meetings with Republican leaders, Bush was playing down the 7:25 a.m. incident, although he said it was "nothing to laugh about."

"We were just driving to work and heard a loud bang," he said. "I just asked what it was and nobody was sure, so we just drove off to work . . . I thought it might have been a shot or something."

Bush said that too much was being made of the incident, that there was "no evidence I know to give it disproportionate attention."

A Secret Service spokeswoman said last night that the investigation into the incident was concluded, but Bush spokesman Pete Teeley said the Secret Service and D.C. police would continue an investigation to determine how the object struck the vehicle.

Officials last night had not determined what had caused the noise Bush heard. A Secret Service spokesman noted that at the time of the incident construction workers in the area were using nail guns, which make a loud popping sound, to drive anchors into building materials.

The intense law enforcement scrutiny surrounding the incident comes amid increased security around Bush and other government leaders after the attempt on President Reagan's life last March and subsequent reports of alleged Libyan assassination plots.

Reporters and cameramen flocked to the scene shortly after the incident was reported, jockeying for camera position in a roped-off area.

"I looked out my window and saw police everywhere," said Rozanne Weissman, a public affairs director who lives at the Savoy Apartments at New Hampshire Avenue and L Street. "I knew something was going on, but didn't know what. I figured some criminal was on the loose."

Hundreds of onlookers gathered behind hastily raised rope barricades and many took to alleys and back building entrances to get to work in their L Street offices.

Bush was going to work at the Executive Office Building, next to the White House, under Secret Service and Washington police escort when he heard the distinct noise. The driver and another Secret Service agent in the front seat of the car could not determine what the noise was and "didn't feel it was a dangerous situation, so they continued on to work," according to a Secret Service spokesman.

Once at the White House complex the agents looked over the car and spotted a tear at the left rear of the vinyl cover of the armor-plated roof. Washington police were notified and the search began.

Shirley Green, a spokesman for Bush, said he appeared "very calm" upon his arrival at work and went about his schedule, which began with an intelligence briefing and an 8 a.m. meeting with former Redskin coach George Allen and Conrad Casey of the President's Commission on Physical Fitness.

Meanwhile some police at the scene were reporting that gunshots were heard and news reporters spent part of the morning chasing down rumors of a gunman on a roof along L Street. Amid the confusion, one television network and an afternoon newspaper reported that the Bush limousine had been struck by gunfire.

The Secret Service cut out the damaged portion of the car roof and sent it to the FBI labratory in the Hoover Building at 10th Street and Pennsylvania Avenue for analysis. By midday it was clear that no bullet had been fired, according to law enforcement officials.

Bob Snow, spokesman for the Secret Service, said that the object had torn the vinyl roof, but not the padding under it or the shell of the armor-plated vehicle. He declined to disclose the cost of the damage to the vehicle.to the vehicle.

Washington Post staff writers Joseph E. Bouchard, Alfred E. Lewis, Loretta Tofani and Benjamin Weiser contributed to this report.