Congressional sources tell FOX News that the letters that showed up at House offices Thursday containing a photo of the Times Square military recruiting office before it was bombed and including the claim "We Did It." may not have gone through the U.S. Mail. And a key Congressman says more letters could be discovered Friday.

The U.S. Capitol Police say that the letters "properly passed through Congressioonal mail screening procedures before arriving at various Congressional Offices and been determined to be safe."

However, FOX News has learned that the letters may have been dropped off in person at internal mail pick-up sites inside the Capitol complex and office buildings, been screened, and then delivered to Congressional offices.

U.S. Capitol police say the envelopes are "affixed with two (one dollar) stamps."

However, sources tell FOX News that it's believed that at least a couple of the letters did not bear postmarks.

Capitol Police say they have not made a concrete link between the letters and the bombing of the military recruitment station at Times Square. But Capitol Hill sources indicate that someone familar with internal House mail procedures could have dropped off the letters in a manner for them to appear at Congressional offices in conjunction with the Times Square incident.

However, it's unclear if the letters could have been dropped off and delivered on the same day.

Since the 2001 Anthrax attacks on the Capitol complex, mail goes through rigorous screening procedures before being delivered to offices. Mail could take as long as four or five days to several weeks to arrive after its been mailed.

In a phone interview with WNYW-TV in New York, the top Republican on the House Homeland Security Committee, Rep. Pete King (R-NY) said more letters could be discovered Friday morning.

"They are expecting quite a few more to be received because I understand there are numbers inside of the letters that indicates there could have been up to 100 of them that have been sent," said King.

Those letters that were discovered appear to have been delivered between 3 and 6 p.m. ET to Congressional offices.

In addition, King described the "document" included in the envelope.

"I've heard first-hand that it's a diatribe type ... like a rant. 20 pages of ranting. And each of the 8 letters had the manifesto enclosed with it."

King did not receive any of the letters.