The FBI is investigating anonymous threats against the Goldman Sachs investment firm that were mailed to newspapers around the country, but does not consider the threats to be of "high credibility," The Associated Press has learned.

The handwritten letters, which were penned in red ink on loose-leaf paper and mailed to 20 newspapers, contained the warning, "Hundreds will die. We are inside. You cannot stop us." The letters were signed "A.Q.U.S.A."

A federal law enforcement official familiar with the investigation told the AP on Friday that the FBI "does not assign high credibility to the threat" because of the circumstances surrounding the letters including their brevity and the nonspecific nature of the threat.

Michael DuVally, a spokesman for Goldman Sachs, said the investment firm was working closely with law enforcement authorities, adding that authorities told the firm they don't believe the threat is credible.

"We take any threat to the safety to our people seriously," DuVally said Friday. "We have a broad range of security measures in place to counter all likely threats and we're monitoring the situation closely."

New York FBI spokesman James Margolin said the bureau was trying to determine the origin of the letters. "All threats are taken seriously," he said.

The letters, which were postmarked in late June from the New York boroughs of Queens and the Bronx, were being analyzed by FBI and U.S. Postal inspectors at the FBI crime lab in Washington, D.C., and at the postal service lab in Dulles, Va., said Tom Boyle, a spokesman for the U.S. Postal Inspection Service.

A second investigator familiar with the case told the AP it may be difficult to isolate the fingerprints of whoever sent the letters because others have touched the envelopes since they were deposited in a neighborhood mailbox.

Investigators also will be looking for DNA evidence that may have been contained in saliva left when the envelopes were sealed. In addition, they will be examining the postal bar codes routinely stamped on letters to pinpoint exactly where the letters entered the mail stream.

The letters were mostly sent to smaller newspapers in Seattle; Boise, Idaho; Corpus Christi, Texas; Fort Wayne, Ind., and Bayonne, N.J. The Star-Ledger of Newark, N.J. is the largest newspaper to have received one of the letters; theirs was postmarked June 27.

The newspapers notified local law enforcement agencies after receiving the letters and in some cases also alerted Goldman Sachs directly, law enforcement officials said.

Goldman Sachs did not receive any of the threatening letters, Boyle said.

Goldman Sachs is based in New York, and has offices in London, Frankfurt, Tokyo, Hong Kong and other cities. About 3,000 people work in its 44-story Jersey City tower.