PHILADELPHIA — A bomb stuffed inside a padded envelope exploded in a downtown apartment early 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 man’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 Tuesday that the envelope contained some type of shrapnel and caused ‘‘substantial damage’’ to appliances in the kitchen. The man’s most serious injury was to his left hand.

The package was ‘‘target-specific,’’ Philadelphia Police Commissioner Richard Ross said. Authorities do not believe the envelope was sent in the mail, because it had an old barcode on it, he said.

Authorities have not seen the type of explosive device in Philadelphia before, Sullivan said, but they are ‘‘familiar with it in other areas of the world.’’

Associated Press