A second St. Mary's County teenager was arrested yesterday and charged as an adult with first-degree murder in the shooting death of a former Great Mills High School basketball standout, the Maryland State Police said.

The alleged triggerman in the murder, Sean Foley Somerville, 18, was arrested Nov. 24 and charged with killing 19-year-old Darryl Ronnell Mackall shortly after a brawl at a nightclub in Callaway. Somerville was ordered held without bond Monday by a St. Mary's County District Court judge.

Maryland State Police investigators said they believe Kirk V. Swales, 17, also of Mechanicsville, the juvenile arrested yesterday, was riding in a car with Somerville when Somerville shot Mackall in the face after a car-to-car shouting dispute on Chancellors Run Road near Great Mills early on Nov. 21.

Mackall was found unconscious and bleeding from the head in his car about 5 a.m. that day after his car had hit a telephone pole along Chancellors Run Road. Police at the scene believed his injuries were from the car accident. However, doctors at Washington Hospital Center later found Mackall had been shot in the head. He died at the hospital later that day. The state medical examiner's office ruled the death a homicide.

Mackall, Somerville, Swales and a number of other local youths allegedly were involved in a fistfight on the evening of Nov. 20 or in the early morning hours of Nov. 21. The youths were playing pool at a local hangout called the Callaway Club, police said.

In an interview with police included in court documents, Somerville admitted to police that he had been involved in a fight at the club but denied involvement in the shooting.

Maj. Thomas Bowers, the commander of the criminal investigative unit of the Maryland State Police, said more arrests were expected.

After the fight at the Callaway Club, Somerville and friends allegedly went looking for Mackall and his associates to "retaliate for the incident in the club," according to a witness's statement. Somerville and his friends then came upon Mackall driving home alone along Chancellors Run Road, the witness said.

The witness told police that the two parties stopped their cars and that Mackall yelled at Somerville and others to "get out and fight."

The witness told police that Somerville then "pulled out a handgun and shot Mackall in the face." The gun was later disposed of by a friend of Somerville's, charging papers said.