WASHINGTON — Police swept across the U.S. Capitol complex to chase a flurry of reports of suspicious packages and envelopes Wednesday after preliminary tests indicated poisonous ricin in two letters sent to President Barack Obama and a Mississippi senator.

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

Reports of suspicious letters also came in from congressional offices in Michigan and Arizona as police investigated the discovery of at least three questionable packages in Senate office buildings.

The rush of activity came as tensions were high in Washington and across the country following the twin bombings on Monday at the Boston Marathon that killed three people and injured more than 170. The FBI said there was 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 have a suspect in mind in the Obama and Wicker mailings, someone 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 possible suspect.

Despite the increased police presence in some areas, normal business continued across most of the Capitol and its office buildings, with tour groups passing through as usual and visitors streaming in and out of Wicker’s office.

Senate Sergeant-at-Arms Terence Gainer said in an email that packages were dropped off at the offices of two senators. 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 man who delivered at least two of the packages was being questioned, Gainer said, as Capitol police swiftly ramped up security and temporarily advised people to avoid parts of the office buildings.

Both the letters to Wicker, R-Miss., and to Obama were intercepted at off-site mail facilities. The FBI said the letters were undergoing further testing. Preliminary testing can be unreliable, showing false positives for ricin.

Gainer told Senate offices they wouldn’t receive mail on Thursday or Friday because of the ongoing investigation and testing. He cautioned congressional offices to be vigilant when handling mail and not to accept sealed envelopes. All unopened items should go through screening, Gainer said.

As the discoveries spread concern, 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. At one point, 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.

Amy Keough of Stow, Mass., and her family were searching for an open entrance to the Russell Senate Office building and walked by a U.S. Capitol Police hazardous materials vehicle. The Keoughs have been visiting Washington for several days, but Monday’s marathon bombing was on their minds.

“We don’t know really what it is that’s going on,” Keough said. “We’re from Massachusetts, so right now anything is possible, with all the events in Boston.”

Sen. Carl Levin 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.

Authorities also are investigating two suspicious letters that were sent to the Phoenix office of Sen. Jeff Flake, R-Ariz., police said. Emergency crews in hazardous materials gear were seen outside the building.

The increased police presence was obvious around the Capitol Wednesday. Outside, many public garbage cans were emptied and turned on their side.

Yet hundreds of people — including House Speaker John Boehner — were gathered in the center of the Capitol rotunda for the presentation of the Congressional Gold Medal to Professor Mohamed Yunnus. The ceremony continued uninterrupted.