A U.S. Park Police officer, making a routine vehicle check in Rock Creek Park near Military Road late Sunday night, was savagely beaten with a piece of lumber that broke his jaw and fractured his skull in several places, officials said yesterday.

The wounded officer, James P. Culver, 24, was discovered about 11:35 p.m. Sunday locked inside his blood-splattered patrol car, which was parked in Picnic Grove 13, at Glover and Grant roads NW, Park Police Maj. William Spruill said.

He was taken by helicopter to Washington Hospital Center, where he was listed in critical condition last night in the intensive care unit after several hours of surgery. A hospital spokeswoman said that Culver, who in addition to the multiple fractures has several bruises on his brain, was drowsy but responding to stimuli.

Culver, whose patrol car had a bloody palm print on the hood and blood dripping down the driver's door, told officers that he was struck from behind while questioning one of the occupants of a red four-wheel-drive vehicle he had found parked in the picnic area, officials said.

Police yesterday arrested a man who surrendered to officers earlier in the day and charged him with assaulting a police officer. Officials last night would not release the suspect's name.

Sources said police also are searching for another man and a woman.

Authorities spent much of yesterday trying to determine exactly what happened Sunday night in the normally quiet area of the federal park.

Sources said yesterday that they were unsure of the time of the attack because Culver, a Park Police officer since Dec. 29, 1988, did not radio in that he was checking out a vehicle.

Officials said Culver, who is assigned to the Rock Creek Station of the Park Police, may have gone to the picnic grove, which officially closed at dark, to retrieve a barricade. "Whatever he was there for was not directly related to the incident," a law enforcement source said.

Culver, who has not been in condition to be interviewed extensively by police, told officers who found him that the first blow was struck from behind. After the attack, Culver managed to get inside his patrol car and radio for help, Sgt. Karl Ore said.

Spruill said that officers who went to the picnic grove "found him inside his cruiser with his doors locked, and of course, they could see that he had been severely attacked, and they got his attention and once he realized that they were police officers, then he rolled his window down."

Culver was able to provide officers with information about the vehicle and its occupants, and authorities quickly put out a lookout for two brothers.

One of the brothers was the man being questioned last night. The suspects' car has not been recovered.

Culver, a native of Pittsburgh who lives in Frederick, Md., had reported to work at 10 p.m. Sunday to work a shift that would have continued until 6 a.m. He had been assigned to the Rock Creek Station for about a year.