WASHINGTON — Letters sent to President Barack Obama and a Mississippi senator that tested positive for poisonous ricin are related and both are postmarked Memphis, Tenn., the FBI said Wednesday. A senator said police have a suspect in mind. Several other reports of suspicious mail to government officials were being checked.

In an intelligence bulletin obtained by The Associated Press, the FBI says that letters to Obama and Sen. Roger Wicker, R-Miss., both say: “To see a wrong and not expose it, is to become a silent partner to its continuance.” Both letters are signed, “I am KC and I approve this message.”

The activity came as tensions were high in Washington and across the country following the deadly bombings on Monday at the Boston Marathon that killed three people and injured more than 170. The FBI said there is no indication of a connection between the letters and the bombing. The letters to Obama and Wicker were postmarked April 8, before the marathon.

Sen. Claire McCaskill, D-Mo., said that police suspect a person who “writes a lot of letters to members.” She made the comment Tuesday as she emerged from a briefing by law enforcement on the Boston bombing. Authorities declined to comment on a suspect.

In addition to the letters, U.S. Capitol police were investigating the discovery of at least three suspicious packages in Senate office buildings.

Senate Sergeant at Arms Terence Gainer said in an email that packages were dropped off at the offices of two senators, and Sen. Richard Shelby, R-Ala., said in a statement his office had received one of them.

A third package was found in an atrium on the first floor of one of the two buildings. A person who delivered at least two of the packages was being questioned, Gainer said, as Capitol police swiftly ramped up security.

Both the letters to Wicker, R-Miss., and to Obama were intercepted at off-site mail facilities.

Separately, Sen. Carl Levin of Mich., issued a statement saying an aide in his Saginaw, Mich. Office had received a suspicious-looking letter. “The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating,” the Michigan Democrat said in a statement.

The discoveries spread concern in the sprawling Capitol complex, and authorities swiftly stepped up their security presence.

In one case, police sealed off a hearing room where Defense Secretary Chuck Hagel and Gen. Martin Dempsey, the chairman of the Joint Chiefs of staff, were testifying.

In another, officers advised Sen. Joe Manchin and aides not to board an elevator because suspicious packages had been found on several floors of the Hart Office Building.

“They just told me there’s something suspicious and they’re looking into it,” Manchin said.

The FBI said the letters to Obama and Wicker were undergoing further testing. Preliminary testing can be unreliable, showing false positives for ricin.

Around the Capitol there was an increased police presence Wednesday. Outside, many public garbage cans were emptied and turned on their side. Yet public tours of the building continued as usual.