A bomb stuffed inside a padded envelope exploded in a downtown apartment Tuesday when a man opened the package, apparently thinking it contained medicine, police said. The victim was hospitalized with hand and chest injuries.

Federal agents and the city bomb squad were investigating the blast that injured a man in his 60s at about 4 a.m. The manโ€™s name has not been released. Authorities said he had arrived home at that hour after being out of town, and was opening his mail.

Chief Inspector Joe Sullivan said at a press conference Tuesday afternoon the envelope contained some type of shrapnel and caused โ€œsubstantial damageโ€ to appliances in the kitchen. The manโ€™s most serious injuries involved his left hand.

The package was โ€œtarget specific,โ€ Philadelphia police Commissioner Richard Ross said. Authorities do not believe the envelope was sent in the mail.

Authorities have not seen the type of explosive device in Philadelphia before, Sullivan said, but they are โ€œfamiliar with it in other areas of the world.โ€

Bureau of Alcohol, Tobacco, Firearms and Explosives special agent Sam Rabadi said the device is being analyzed an ATF laboratory.

The victim was taken to Thomas Jefferson University Hospital and was in stable condition after surgery.

The man opened the package near the stove in the kitchen of the apartment he shares with a roommate, leading police to initially believe the blast may have been an accidental explosion caused by an inhaler getting too close to heat.