Federal and local authorities were continuing Wednesday to try to determine who sent a package that exploded in a Center City apartment this week, injuring a 60-year-old man.

Christopher Curran, special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives, said investigators were still conducting interviews and awaiting tests on the device, which detonated around 4 a.m. Tuesday when it was opened by Jim Alden inside his apartment on the 1800 block of Pine Street.

Police have said they believe Alden was the target but have not identified any suspects or motive.

Alden, described by friends as a gregarious man with few obvious enemies, was not listed as a patient Wednesday at Thomas Jefferson University Hospital, according to a spokeswoman. Police said Tuesday that Alden had been in stable condition there while recovering from injuries to a hand and his face and chest.

Attempts to reach Alden on Wednesday were unsuccessful.

Authorities said Tuesday that the explosive device was delivered in an envelope to Alden's home, though they were not certain when it was dropped off. They also did not specify whether it had been sent through the mail or personally delivered.

The device was set to detonate when the envelope was opened, officials said. Sam Rabadi, special agent in charge of the ATF Philadelphia field division, said Tuesday that the device was "a little different" than those the agency typically sees, and that it had been sent to an ATF lab for testing.

Police said they do not believe the attack had any connections to a terrorist organization, and added there was no obvious indication that Alden - who lives with a male partner - had been targeted for his sexual orientation or political beliefs.