WASHINGTON (AP) — The U.S. Secret Service has intercepted a letter addressed to President Barack Obama that contained a "suspicious substance."

A law enforcement official said the letter is very similar to one recently mailed to Mississippi Republican Sen. Roger Wicker. That letter tested positive Tuesday for poisonous ricin.

The official requested anonymity because the official was not authorized to discuss the ongoing investigation.

The letters were received at separate facilities that sort mail addressed to the White House and Capitol Hill. The mail facilities are not located on the main White House and Capitol Hill complexes.

Word of the suspicious letters comes amid already heightened tensions in Washington and across the country since the deadly bombings on Monday at the Boston Marathon that killed three people and injured more than 170. Law enforcement officials haven't said whether they believe the letters are related in any way to the Boston bombings.

Secret Service spokesman Ed Donovan said the letter to Obama was intercepted Tuesday, the same day congressional officials alerted the public to the letter sent to Wicker. Secret Service is working with the FBI, as well as U.S. Capitol Police, on the investigation, Donovan said.