It looked innocuous enough, the large envelope that had been dropped outside Jim Alden's apartment near Rittenhouse Square.

But when Alden opened it around 4 a.m. Tuesday - apparently thinking it contained medication - the package exploded, police said, leaving the 60-year-old hospitalized with injuries to a hand and his face, and authorities searching for answers in an incident that seemed ripped from the pages of a spy novel.

"This is not something we see every day in our city," said Sam Rabadi, special agent in charge of the Philadelphia field division for the Bureau of Alcohol, Tobacco, Firearms, and Explosives.

Federal and local authorities on Tuesday were still searching for clues about who sent the explosive package and why, saying they had not uncovered a motive or identified any suspects.

But during a news conference at the ATF field office in Center City, they said that the incident apparently was not the work of anyone tied to a terrorist organization - and that the package seemed targeted for Alden.

That stunned Alden's friends, who described him as a gregarious theater buff and singer who worked for a time as a waiter and bartender at the Warwick Hotel Rittenhouse Square.

"I can't think of anything in his life [that would] make him a potential target for anybody," said Doreen Stoller, who said she was "flabbergasted" by the apparent attack.

Alden lived with a partner, police said, and had a rainbow "Love Trumps Hate" sign in the front window of his building, on the 1800 block of Pine Street.

But investigators said there was no obvious indication that the incident was motivated by either Alden's sexual orientation or his politics. Without a suspect, they were not ruling anything out.

"We are going to look at every possible motive that comes across our radar," Rabadi said.

Rabadi and other officials, including Police Commissioner Richard Ross, Chief Inspector Joe Sullivan, and representatives from the FBI and the U.S. Postal Service, said the explosive detonated when Alden opened the package in his kitchen.

The device had been in an envelope that may have been dropped off rather than sent through the mail, they said, although it was not immediately clear when or how the package had been delivered.

Rabadi said authorities sent the device, designed to detonate upon being opened, to the ATF lab for testing. He described it as "a little different" from explosives the agency typically sees, but declined to elaborate.

The worst of Alden's injuries was to a hand, officials said, but he also suffered injuries to his face and chest. He was reported in stable condition after surgery Tuesday at Thomas Jefferson University Hospital, officials said.

Authorities were searching the area for surveillance video and other evidence that might lead to a suspect.

"We will bring every resource to bear to try and figure out what actually happened here," Rabadi said.

Friends said Alden studied music at California State University, and for a while was in the New York cast of Forbidden Broadway, an Off-Broadway revue that parodies musical theater.

More recently, said a friend who asked not to be identified, Alden had been working at the Warwick, where he had been a banquet waiter and "amazing bartender" in addition to being "a fantastic guy."

Gato Espinel, who worked for Alden at the Warwick in dining services, described him as a "well-mannered, very nice guy who is supersensitive about not saying the wrong thing to people."

Stoller, executive director of the nonprofit Hermann Park Conservancy in Houston, knows Alden from her friendship with David Olive, a former partner of Alden's who is deceased. She said that Alden possesses a life force that draws people in, and that he could always be coaxed into singing show tunes, even in the Warwick lobby.

"Jim is kind, animated, and friendly," she said, adding that the apparent attack on Alden is a frightening puzzle.

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Staff writer Mari A. Schaefer contributed to this article.