Second Philly 'bomb' a false alarm

A police bomb squad finds the bomb scare near a mailbox in Philadelphia on Tuesday was a false alarm.

PHILADELPHIA, Pennsylvania (CNN) -- A day after a bomb was found and detonated in a Philadephia mailbox, an apprehensive bomb squad investigating suspicious items near a second mailbox found only a pair of sneakers with a note saying "Free Palestine" inside, a police officer said.

The sneakers were found in northeast Philadelphia at 11:21 a.m., said Officer Maria Ibrahim. The police bomb squad went to the scene because concern in the city was heightened after Monday's explosives device was found.

FBI Special Agent Linda Vizi called Tuesday's incident a "false alarm."

On Monday, a message attached to a bomb left in a northeast Philadelphia mailbox also said "Free Palestine" and mentioned al Qaeda, the terrorist network of Osama bin Laden, local and federal law enforcement sources told CNN.

The FBI in Washington will analyze evidence in the case, according to Vizi.

A mail carrier discovered a suspicious package in this mailbox in northeast Philadelphia that contained a bomb.

She said agents are trying to gather leads as to who left the package in the mailbox Monday and who its target might have been. The bomb detonated as authorities tried to defuse it. No one was hurt. At this time, investigators do not believe the al Qaeda terrorist network was involved.

The bomb, which was found in a foam box, was left in a mailbox on a street corner and was probably placed there sometime between early Sunday evening and midday Monday. Vizi would not disclose details of the bomb itself.

Authorities were quoted in The Philadelphia Inquirer newspaper as saying the device was more sophisticated than a pipe bomb. Sources told the paper the bomb "was wrapped in a foam box packed with nails and had a sophisticated switch to allow the bomber to plant the device safely without it exploding prematurely."

The package was not addressed and did not have any postage, federal and local investigators told CNN. The name "al Qaeda" was misspelled, investigators said.

One investigator rated the bomb in terms of sophistication at a "six ... 10 being the most sophisticated."

"Usually, in mailboxes, you find a pipe bomb or a plastic liter bottle with carbon dry ice. This was different, it was a place device," he said. The device had items in it that could cause harm once the bomb exploded.

Investigators noted that the person who placed the bomb in the collection box did not go out of his or her way to hide it. It was also made, according to investigators, to withstand jostling, and was unlikely to go off if just picked up.

Vizi said she is "very concerned about copycats."

Pipe bombs were found hidden in rural mailboxes in several states throughout the country earlier this month, sparking at least one other incident of a copycat bomb.

After the bomb detonated Monday afternoon as bomb squad officers worked on it, investigators said it was a strong device, sending shrapnel about 100 feet from the blast site.

Vizi would not discuss what writing was on the package, but said it made the mail carrier suspicious enough to call police, who dispatched a bomb disposal unit to the site.

"The mail carrier went to retrieve the mail out of the mailbox, and he noticed a package that did not look right," Vizi said. "He did not like the way it looked, he did not like the writing on it."