PHILADELPHIA (AP) _ Gunmen jumped from a car and opened fire in a daytime ambush Thursday, killing the brother of the Mafia's Philadelphia underboss and shooting another mob scion in the buttocks three times, authorities said.

The attack represents a resurgence of a three-year power struggle within the Philadelphia-South Jersey mob that had pitted the victim, Michael Ciancaglini, against his older brother, authorities said.

Ciancaglini (pronounced Ching-a-lini) and Joseph Merlino were walking through the heart of South Philadelphia's mob stronghold around 1:30 p.m. when two gunmen wearing baseball caps drove up in a white car and fired 10 to 15 shots with semiautomatic or automatic guns, said Richard A. Zappile, chief of detectives.

The gunmen then got back in the car and fled. No arrests have been made.

Ciancaglini, 31, died of wounds to the chest and lungs. Merlino, 31, was in stable condition at Pennsylvania Hospital, spokeswoman Lisa Chapman said.

The attack came five months after three masked men entered the diner run by Ciancaglini's 36-year-old brother, Joseph, and shot him three times in the head, once in the foot and once in the shoulder. He survived.

Fred Martens, executive director of the Pennsylvania Crime Commission, said Michael Ciancaglini's slaying undoubtedly came in retaliation for his brother's shooting March 2.

Later Thursday, police found a white car with New Jersey tags afire in another section of South Philadelphia. They were investigating its ties to the shootings, Police Commissioner Richard Neal said.

Al Arena, 74, was sitting on a lawn chair about 200 yards away from Thursday's shooting, just blocks from the city's Italian Market.

''I said, 'Who the hell's throwing a firecracker?''' he said. ''The next thing you know, somebody's hollering. The one guy was sprawled out and bleeding pretty bad. His face and arm were full of blood.''

Shell casings littered the cordoned-off block as spectators gathered to watch police investigate. Nearly a dozen people interviewed said they neither saw nor heard anything.

Just 1 1/2 years ago, Michael Ciancaglini escaped an attack blamed on the mob struggle when someone fired five shotgun blasts at his home.

Thursday's shooting was the latest in at least seven mob attacks as two factions struggle for control of an organization left adrift when boss Nicodemo ''Little Nicky'' Scarfo was sent to prison on a racketeering conviction in 1989.

Both Michael Ciancaglini and Merlino were soldiers in a faction that remained loyal to Scarfo when the Sicilian-born John Stanfa took over the family about eight months ago, Martens said.