The man in custody, believed to have been the driver, was tackled by Livewell's partner. Detectives had not charged the man Friday night and declined to identify him.
"We're not putting anything out," a police spokeswoman said, adding that investigators would not release the van driver's name until the other two men were in custody. Homicide detectives were running the investigation.
Deputy Police Commissioner Richard Ross described the collection of weapons recovered at the scene as "absolutely scary."
The fleeing men dropped two assault rifles - an AK-47 and an SKS carbine - as well as a Glock pistol. Inside the van were two Bushmaster AR-15 assault-type rifles, a TEC-9 semiautomatic pistol, a Taurus PT24 pistol, and another Glock.
Except for the Glocks, the weapons can fire bullets "capable of slicing through a vehicle" or piercing armor, Ross said.
"We don't know what they intended to do, but it was no good," Police Commissioner Charles H. Ramsey said. "To have this kind of armament is an indication they intended to harm someone."
Ramsey pointed out that an SKS carbine was used to kill police Sgt. Stephen Liczbinski in May 2008. Two men are on trial in his death.
All of the guns are legal to purchase in Pennsylvania, said Lt. Vincent Testa, head of the firearms investigation unit.
The officers were acting on "information" in pulling the van over, Ramsey said. He declined to confirm a report that the stop was related to an earlier incident.
"We don't know if there's any connection to any other shooting that might have taken place," he said.
Livewell, a nine-year veteran of the department, is married and has a 15-year-old daughter and an 8-month-old son.
Ramsey visited Livewell at the hospital and said he was "in good spirits and doing well."
Anyone with information is being asked to call 215-686-3334 or -3335, or the Police Department's anonymous tip line, 215-686-8477.
Contact staff writer Peter Mucha at 215-854-2443 or firstname.lastname@example.org.
Inquirer staff writer Sam Wood contributed to this article.