BALTIMORE, OCT. 15 -- A 23-year-old prison escapee was indicted by a federal grand jury today in the shooting of a U.S. Park Police officer on the Baltimore-Washington Parkway Aug. 21 that triggered a mammoth manhunt and traffic jam on the parkway and nearby Capital Beltway.

Kendall Charles Alexander, a convicted robber who had recently escaped from a Maryland prison, was charged with assault and related counts in the incident in which police said Park Police Officer Patrick Gavin was shot once in the head after stopping a car occupied by Alexander and two other persons. The car was later determined to have been stolen.

All three men fled the shooting scene on foot but were arrested later. Gavin, bleeding from his left temple, was able to fire once at the fleeing men and called for help on his radio, police said. He was taken to the Washington Hospital Center, treated and released several days later.

Alexander, of Baltimore, who has been returned to prison to resume his sentence for robbery, was charged in today's indictment with assault on a federal officer, assault with intent to murder, being a felon in possession of a firearm and other counts.

If convicted, he faces a maximum possible sentence of 65 years' additional imprisonment.

The other men arrested in the shooting are Vernon Sykes, 22, and Antonio L. Johnson, 26, both of Baltimore. Both have been charged but not indicted in the Gavin shooting. A source close to the investigation said at least one of the two is cooperating with prosecutors in the case against Alexander, who police identified as the triggerman.

The incident began about 2:30 p.m. Aug. 21 after Gavin, a three-year member of the Park Police, pulled over a car in what police called a routine traffic stop on the parkway between Riverdale Road and the Capital Beltway.

Police said Gavin walked to the car and then returned to his own cruiser and got in. At that point, police said, the driver of the car got out and approached Gavin, spoke to him briefly and then shot him in the head with a small-caliber pistol.

Gavin radioed for help, and a search involving up to 100 officers began immediately. Afternoon rush-hour traffic was tied up for several hours. Sykes and Johnson were captured near the scene, according to police, and Alexander was arrested nine days later when an anonymous telephone tip led police to him in Baltimore.

"We intend to prosecute Kendall Charles Alexander to the maximum extent of the law," said Maryland U.S. Attorney Breckinridge L. Willcox. "We shall do the same to other criminals who shoot police officers."