WASHINGTON — A letter intended for President Obama has tested positive for the deadly poison ricin, one day after a poison-laced envelope was sent to Sen. Roger Wicker.

The second letter was received at an offsite mail screening facility and was immediately quarantined by the Secret Service, the FBI said in a statement.

The FBI says the letters sent to Obama and Wicker are related and are both postmarked out of Memphis, Tenn., dated April 8.

In an intelligence bulletin obtained by The Associated Press, the FBI says the letters 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.”

Sources told the Post both letters have tested positive for ricin in two of three tests.

The Secret Service said they are working closely with US Capitol Police and the FBI’s investigation.

The FBI says there is no indication of a connection to the bombing at Monday’s Boston Marathon.

White House press secretary Jay Carney confirmed that a suspicious letter was headed to the president before it was intercepted.

“There was a letter sent to, addressed to, the president at an off-site mail facility [that] was noticed to contain a suspicious substance and tests were undertaken,” he said.

“These investigations, both of the bombings in Boston and the letters in question here, are just underway,” Carney said.

“I would point to the statement of the FBI … that says they have no indication of a connection to the two.”

Meanwhile, police swarmed the Hart and Russell Senate Office Buildings. For 90 minutes, cops told Senate employees to stay in their offices cleared the hallways of any visitors.

The lockdown lasted for 90 minutes before cops allowed employees to leave and visitors to enter again.

Sen. Richard Shelby’s office told NBC that Capitol Police are investigating a suspicious package at his office in the building. Michigan Sen. Carl Levin said he also received a suspicious letter at Saginaw regional office, but further details were unavailable. A source told the Post both packages have not yet undergone the preliminary round of tests.

Capitol Police confirmed today that they are investigating two suspicious items and are “controlling access” to parts of two Senate office buildings and the Capitol. It is unclear if they are related to the White House ricin scare.

Authorities last night said they had identified a suspect who may have sent the letter addressed to Sen. Wicker, although he did not appear to be in custody.

The letter was intercepted by an off-site mail-screening facility in DC yesterday afternoon and was later sent to a testing facility in Maryland.

Senate Sergeant at Arms Terrance Gainer told senators that the letter had been postmarked in Memphis, Tenn., and carried no return address and no exterior markings making it suspicious.

Sen. Claire McCaskill (D-Missouri) said a suspect has already been identified but didn’t address whether he had been arrested. She said the letter was from someone known to frequently write lawmakers, but didn’t give the person’s name.

Ricin has previously been used in assassinations. Just a tiny amount can be deadly. In 2003 and 2004, several letters laced with ricin were intercepted on their way to the White House and the Dirksen Senate Office building.

Investigators are looking at the timing similarities between this attack and 9/11, when in the midst of the confusion surrounding the al Qaeda attack, anthrax letters were dispatched to government and media offices, including The Post.

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