Police and federal authorities are searching for the motive behind a manila envelope being sent to a Center City man that detonated and flung shrapnel into his body, seriously injuring him.

The victim, whom authorities did not identify, was hospitalized in critical condition. He is reportedly a 60-year-old man who is active in the gay community and had a rainbow sign in his window with the words "Love Trumps Hate."

Authorities said they do not know why the the unknown bomber targeted the victim, who lives on the 1800 block of Pine Street.

"It was constructed so when you open the envelope, it detonated," said Sam Rabadi, Special Agent in Charge of the Philadelphia ATF field office, at a news conference on Tuesday. "Upon being opened by the victim, it exploded."

Police were called at 4 a.m. Tuesday with reports of an explosion at the victim's home. The victim is hospitalized at Jefferson University Hospital with wounds from shrapnel to his hand, torso and severe injuries to his left hand, police said. 

The explosion caused "substantial damage" to the victim's residence and appliances nearby, but didn't cause structural damage to the building, Chief Inspector Joe Sullivan said at the news conference.

"We wanted to assure the community that this is an isolated incident. There is one device," he said. "They should feel safe and secure tonight."

Authorities do not believe the incident was terrorism-related, saying it appeared that the bomb was specifically intended for the victim who opened it.

There is "no nexus to suggest it's related to any particular group or any ideology," Philadelphia Police Commissioner Richard Ross said.

Authorities do not yet know if the package was transported through the U.S. Postal Service or if it was placed at the victim's home by the bomber.

Police said the victim had just returned from being out of town for a few days, and thought the envelope contained medication when he opened it.

Anyone with information about the source of the explosive is being urged to contact police.