By Peter Eisler, Kevin Johnson and Aamer Madhani

Federal authorities are investigating a letter containing a “suspicious substance” that was addressed to President Obama, the agency confirmed Wednesday, and at least four U.S. senators also reported receiving suspicious mail. FBI spokesman Paul Bresson said preliminary tests on the letter to Obama showed evidence of ricin, a powerful toxin that is generally considered to be ineffective as a weapon for mass terrorism.The letter was sent to Obama on Tuesday, and was intercepted at the White House mail screening facility, according to Secret Service spokesman Brian Leary.The screening facility is a remote facility, not located near the White House complex, Leary said. The revelation of the suspicious letter to Obama comes less than 24 hours after U.S. Capitol Police confirmed it was investigating a letter addressed to Sen. Roger Wicker, R-Miss., that tested positive for the poison ricin in a preliminary examination.Obama was briefed on the suspicious letters Tuesday night and again Wednesday morning, said White House press secretary Jay Carney.The FBI says there is no indication that the suspicious letters are connected to this week’s twin bombings at the Boston Marathon. Carney cautioned that Americans shouldn’t jump to any conclusions.”Before we speculate or make connections that we don’t know…we need to get the facts,” Carney said.

Read More Initial tests show ricin in letter to Obama.