WASHINGTON (CBSNewYork/AP) — A letter that may have contained ricin or another poison was sent to the office of Sen. Roger Wicker of Mississippi.

CBS News reported that the letter preliminarily tested positive for ricin poison, but that further testing is required. CBS News has also learned that results from the more definitive test to be taken at the Baltimore FBI office will not be available until Wednesday.

A source told CBS News that an initial test on the letter returned a “slight positive” while two other tests were negative for radiological or biohazard material. A source said it’s common for false positives to come back on field tests of ricin, CBS News reported.

There was no further information on who sent the letter or what message may have accompanied the letter, CBS News reported. The Senate Sergeant at Arms said the letter had no return address but was postmarked Memphis, Tenn.

Also, CBS News has learned that the letter was sent the first week in April.

Wicker confirmed Tuesday night that the letter had been sent to his office.

In brief remarks to reporters in the Capitol, Senate Majority Leader Harry Reid (D-Nev.) did not say when the letter was sent. It’s customary for mail addressed to congressional offices to be screened at an off-site facility.

Senator Claire McCaskill (D-Mo.) also confirmed a letter containing poison had been sent to a senator. She said lawmakers were informed of the fact at a closed-door briefing about the Boston Marathon bombings.

McCaskill told reporters that at the closed-door meeting, the senators were told the suspected letter writer writes a lot of letters to members. She added she believes authorities have someone in custody, CBS News reported.

McCaskill also said the House and Senate post offices have been temporarily shut down, CBS News reported.

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