"No terrorist group has owned up to it. There is no indication at this point that it was terrorist related," said John Sinnen, assistant special agent in charge of the U.S. Postal Inspection Service in Philadelphia.
The device, which contained a foam box packed with nails and an explosive charge, was detonated Monday when a police bomb squad shot the package with a water cannon. The blast hurled shrapnel 100 feet, but injured no one.
Police and federal agents on Tuesday were investigating a second suspicious package found just before noon in another Northeast Philadelphia mailbox. But FBI spokeswoman Linda Vizy said the package wasn't a bomb.
Investigators described the device found Monday as more sophisticated than the series of crude pipe bombs that injured six people in the Midwest earlier this month, but said it was not immediately clear whom the bomber intended to harm.
The package carrying the bomb, found by a letter carrier Monday in a Northeast Philadelphia mail drop, was not addressed and bore no postage. In addition to the note, which also mentioned the al Qaeda terrorist network, the parcel had protruding wires almost certain to raise suspicion, Sinnen said.
"Luckily enough, this guy didn't make much of an attempt to disguise it," he said.
The note and wires instantly caught the attention of letter carrier Matthew Widmeier, who found the package at about 12:30 p.m. and immediately phoned his supervisor, police and postal inspectors.
Postal workers received a warning Tuesday to be on the lookout for packages, said U.S. Postal Service spokeswoman Belinda Kelley.
"We have had ongoing talks since September 11th to remain vigilant and take caution, and if they see a suspicious package not to touch it," Kelley said.
Widmeier finished his mail route after discovering the bomb Monday and was back at work Tuesday, Kelley said.
Sinnen said investigators didn't have a suspect but were exploring several theories, including the possibility that the bomb might have been placed by a "copycat" inspired by the Midwest attacks, in which a 21-year-old art student from Minnesota has been charged.
"It certainly could be that — a publicity stunt. It could be someone who wants that sort of attention. We aren't sure," Sinnen said.
Inspectors planned to interview people who deposited letters in the box around the time the bomb was left to see if they noticed anyone trying to mail a package.
Authorities were considering the incident an act of terrorism, even if the person who placed the bomb has no political agenda, said Vizi, the FBI spokeswoman.
"The fact of the matter is that it makes people afraid to go to their mailbox, just like it makes people afraid to go to a coffee shop in Israel," Vizi said.