The District has decided not to press charges against a Howard University student who accused a plainclothes police officer of assaulting her over a pedestrian infraction.

The attorney for Chandra Shealey, who was charged with resisting arrest and illegally crossing a street, said six witnesses were prepared to testify that the officer jumped her on Aug. 22.

The case was set to be heard Wednesday, but earlier this week the District's corporation counsel decided not to press charges, said Shealey's attorney, Nathaniel Speights. City officials did not return telephone calls yesterday, and the reasons for the decision were unknown.

The case drew the attention of the local chapter of the American Civil Liberties Union, which at one point offered to represent Shealey. Police, who have declined to comment on the incident, said yesterday the case is still under internal investigation.

Shealey, 19, said she was crossing the intersection of 15th and Euclid streets NW about 7:30 p.m. when she heard a comment she couldn't understand coming from the direction of white car. She said she ignored it, and kept walking.

After hearing a second yell, Shealey said she turned around and saw a man wearing jeans, a T-shirt and sneakers running in her direction. She said she ran and was tackled from behind by the man, who she insists never identified himself as a police officer.

It was only when a uniformed officer intervened, Shealey said, that she learned the man was a plainclothes investigator, later identified as Wayne Simpson.

Police officials later said Simpson, who is assigned to the vice unit, was in a marked patrol car. That conflicts with the account provided by Shealey and several witnesses, who told Speights that they saw only a white car in the area.

"When she first saw him, he was on the street, so the car had nothing to do with it," Speights said. "It's irrelevant to the case."

Police officials would not discuss the specifics of the case or the internal investigation.

The regulations on traffic enforcement stipulate that "only on-duty uniformed members driving marked departmental vehicles shall take enforcement action." Only when a violation "that is so grave as to pose an immediate threat to the safety of others" can an off-duty, plainclothes officer take action, the regulations said.

In a section dealing solely with pedestrians, the guidelines say only that "uniformed members shall enforce the traffic regulations." Summary arrests can be made by these officers "only when the violator refuses to properly indentify himself or otherwise creates a situation whereby the member is left with no alternative . . . . "