A man was injured early Tuesday when he opened a package containing an explosive device in his Center City apartment, police said.
Police Commissioner Richard Ross said the victim appeared to be the intended target.
Ross said the bomb squad described the explosive as a "victim activated device" that was sent specifically to the 60-year-old man, who triggered it when he opened the package in his home on the 1800 block of Pine Street.
Police at the scene put the time of the explosion at around 4 a.m.
The victim apparently thought the package contained his asthma medication, Ross said. A motive was not immediately clear.
Neighbors and a law enforcement source identified the victim as Jim Alden, who lives in the front, ground floor apartment in the building.
Ross did not know specifically what the explosive was made from, but said it caused injuries to the victim's face, chest, and hands, and that it could have started a fire inside the building.
The victim, whom Ross did not identify but said worked as a caterer, was reported in stable condition at Jefferson University Hospital.
Police believe the package was not sent through the mail because the bar code on it was old, Ross said.
"Someone placed it there," he said.
Police believe the package could have been delivered over the weekend, but the victim did not open it until overnight Tuesday, Ross said.
A federal law enforcement source said the device had a power source and a loop switch and that there was the smell of sulphur after the explosion.
Another man was inside the apartment when the explosion went off but was not injured, Ross said.
Investigators were conducting interviews along the tree-lined block and checking for area surveillance cameras to determine who might have sent or delivered the bomb.
Federal agencies have joined the investigation, including the U.S. Postal Service and the Bureau of Alcohol, Tobacco and Firearms .
Hours after the explosion, investigators remained at the townhouse, which has a rainbow "Love Trumps Hate" sign in the front window.
Alden, the apparent target of the bomb, was born in Modesto, Calif., and was enamored of the theater from an early age, according to friends.
He studied music at California State University, said Doreen Stoller. "He has a voice that's just gorgeous," said Stoller, executive director of the Hermann Park Conservancy in Houston.
Alden's ardor took him to New York City, where he was, for a while, in the cast of "Forbidden Broadway," an off-Broadway revue that parodies musical theater, said a friend who asked not to be identified.
More recently, Alden has been working at the Warwick Hotel Rittenhouse Square, the friend said.
Staff writer Mari A. Schaefer contributed to this story