The FBI has confirmed that a letter addressed to President Obama has "preliminarily tested positive" for ricin, a day after lawmakers said another letter sent to the Capitol Hill office of Sen. Roger Wicker tested positive for the same substance.

The warnings come amid a flurry of reports on suspicious packages. Fox News has learned of several suspicious packages or envelopes in various Capitol Hill office buildings. Capitol Police say three packages that were flagged have been removed and the areas have been cleared.

Separately, the office of Sen. Carl Levin, D-Mich., reported a "suspicious-looking letter" at one of the senator's Michigan offices; and a spokesman for Sen. Jeff Flake, R-Ariz., reported a "suspicious letter" was intercepted at Flake's Phoenix office. Authorities were also called to the Dallas office of Sen. John Cornyn, R-Texas, over what was described only as "a piece of mail" -- but field tests later came back negative.

The package alerts compounded security concerns in the wake of the letter incidents and the Boston bombing, which the FBI has said do not appear to be related.

Both of the initial suspicious letters were apparently intercepted on Tuesday. They never reached the Hill or the White House.

An FBI bulletin obtained by Fox News said the letter sent to Obama is still being screened, though it initially tested positive.

The bulletin said both letters included the phrase: "To see a wrong and not expose it, is to become a silent partner to its continuance."

Both were signed, "I am KC and I approve this message."

The letter to the Mississippi Republican was intercepted at an off-site mail screening facility.

Authorities declined to comment on a suspect or any other aspect of the investigation being led by Capitol Police, but Sen. Claire McCaskill, D-Mo., told the Associated Press police have a suspect in mind.

"The person that is a suspect writes a lot of letters to members," McCaskill said as she emerged from a classified briefing.

Testing is apparently still underway. The letter tested positive for ricin in a field test, but the FBI released a statement Wednesday saying tests have shown "inconsistent results" and the substance is being further analyzed.

According to a Homeland Security Department handbook, ricin is deadliest when inhaled. It is not contagious, but there is no antidote.

Wicker thanked law enforcement officials in a statement for "their hard work and diligence in keeping" those who work in the Capitol safe, adding that the matter is part of an ongoing investigation by Capitol Police and the FBI. "Gayle and I appreciate everyone's thoughts and prayers," he said.

As of Tuesday night, mail delivery had only been stopped to the Senate, not the House.

"It is of concern," Sen. Angus King, I-Maine, said after learning about the incident in a briefing with other senators late Tuesday.

The envelope had a Tennessee postmark and no return address.

The letter inside included an implied threat to effect of: "You haven’t listen to me before. Now you will, even if people have to die," Politico also reported.

Sources say officials are familiar with the person believed to have sent the letter as the person has sent other letters before.

FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano were on Capitol Hill on Tuesday for a scheduled talk about cyber security. But that briefing morphed into talks about Boston, after the bombings Monday.

Senate Sergeant at Arms Terry Gainer conducted a separate briefing for senators specifically on ricin.

It's unclear whether the letter had any connection to the Boston attack.

The mail-screening system was established after the Anthrax attacks of 2001 that closed the Hart Senate Office Building.

Fox News' Chad Pergram and Mike Levine and The Associated Press contributed to this report.