PITTSBURGH -- A 17-year-old boy was arrested Friday in connection with a double-shooting that killed a Carrick High School student.

Derwin Milligan, of Allentown, was charged with criminal homicide and taken to the Allegheny County Jail early Friday evening.

Police will not say whether Milligan is cooperating with the investigation. But in the meantime, police brouhg in a dog trained to find weapons, hoping to recover the weapon used to kill Keith Watts.

Pittsburgh police say the investigation is continuing and they expect to make more arrests.

Keith Watts, 16, of Knoxville, was slain in a drive-by while sitting behind the wheel of a parked vehicle about a block from school on Wednesday afternoon.

Alfred Grimmitt, 17, was also shot, but survived. A third teenager was either wounded by shattered glass or grazed by a bullet.

Earlier this week, police cited feuding between factions in the city's Beltzhoover and St. Clair Village neighborhoods as a possible motive for the shootings.

Children from both of the quarreling areas attend Carrick, and Cmdr. Maurita Bryant said making an arrest was especially important because police were hearing street talk of possible retaliation.

Milligan was privately identified as a prime suspect soon after the shootings, according to police.

On Thursday, police complained about being unable to arrest their suspect becuase no witnesses would finger him. It's unclear what changed since that time.

According to a news release, city homicide detectives are being assisted by agents from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. The release did not give a reason for ATF involvement, but investigators believe that an assault-style rifle was used in the shootings.

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