But when the friend arrived at the house on the 2400 block of South 72d Street early Wednesday, he was robbed, bound, gagged, and stabbed alongside the brothers, sources said.
In the predawn hours, sources said, all three men, still alive, were loaded into a van and driven to a parking lot on Kelly Drive near the rowing grandstands on the Schuylkill.
One by one, they were brought out of the van and stabbed again on the riverbank, sources said. The two brothers' throats were slit, and their legs were chained to buckets filled with tar before they were tossed into the shallows.
The 23-year-old was stabbed multiple times, but his captors did not weigh him down. He played dead, police said, and when his attackers left, he managed to climb out of the river and stagger - bloody, and wearing only his underwear - down Kelly Drive, where he flagged down a passing patrol car about 4 a.m.
Police found the brothers in the water shortly afterward. They were pronounced dead at the scene.
Homicide Capt. James Clark said police were searching for five to six men they believed were involved in the killings.
"It's one of the more barbaric murders I've seen," he said.
On Wednesday night, U.S. marshals used a battering ram to break down the door of a house on the Southwest Philadelphia block where police believed the men had been held before their deaths. Detectives searched the house for evidence, along with a trailer parked next to an apparently abandoned house across the street.
The house that detectives searched was empty, but police seized six dogs from the premises. A neighbor said a middle-aged man lived in the house with his wife and young children. The neighbor said that people frequently came and went from the house and that the middle-aged man had been seen coming and going from the abandoned house across the street from time to time.
The man who survived the attack has a criminal record and is well known to police, Clark said. He has also been targeted in the past, according to law enforcement sources.
In July, the man filed a complaint with police saying that an acquaintance had scratched his car, then told him to meet him in Chinatown, adding that he had a gun, according to a police report.
Then, on the night of Aug. 9, he was pulled from his car and attacked with a hammer in Southwest Center City, according to a police report.
The men who attacked him, he told police, told him to "get in their car before they kill him." He ran away and called police, according to the report. Two men were arrested in connection with the incident.
Clark did not identify any of the victims. All were originally from Vietnam, a law enforcement source said.
One, a law enforcement source said, lived on the 700 block of North Second Street. A crime-scene unit was there Wednesday afternoon.
Investigators originally believed that they had spotted blood on the doorknob of the victim's second-floor apartment and that the man may have been abducted from his home, a law enforcement source said.
But crime-scene investigators determined that the substance on the doorknob was not blood, according to the source.
The men had not been abducted from the apartment, a source said.
Police urged anyone who may have witnessed the incident or had information about the case to call the homicide unit at 215-686-3334.