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The feds are probing whether Jussie Smollett was involved in sending a suspicious threat letter to himself a week before he claimed to have been attacked by racists with a noose and bleach in Chicago, according to a report on Tuesday.

The FBI and US Postal inspectors are now investigating the claim by brothers Abimbola and Olabinjo Osundairo that Smollett played a role in the letter delivered to Cinespace Studios on Jan. 22, two federal officials confirmed to ABC News.

The note — which featured the message “You will die black f-g” in cutout magazine letters and a crude drawing of a stick figure hanging from a tree — is at an FBI crime lab for analysis.

“MAGA” was written in red across the envelope. The letter also contained a white powdery substance later determined to be aspirin.

Revelation of the federal probe came as sources told ABC that the Osundairo brothers told investigators that Smollett hatched an idea to stage the Jan. 29 attack because he was upset over the lack of attention generated by the threatening letter.

Police are investigating those allegations, and the brothers — who were arrested Feb. 13 but released without charges two days later — were spotted with their lawyer at the Leighton Criminal Court building on Tuesday, ABC reported.

The brothers met with cops and prosecutors at the courthouse, according to Chicago police spokesman Anthony Guglielmi, amid reports that a grand jury has been seated, possibly to investigate whether the attack was a hoax.

Reports have said Smollett paid the men $3,500 up front to fake the attack and was supposed to give them another $500 afterward.

A grand jury was set to begin weighing charges against Smollett as early as Tuesday, TMZ reported. He could face a felony rap if he is charged with filing a false police report.

The Osundairo brothers are expected to testify before the grand jury if they haven’t already done so, sources told CBS News.

Meanwhile, cops also got a tip Tuesday morning that Smollett and the brothers were spotted together around the time of the alleged 2 a.m. attack on the “Empire” actor, Guglielmi told The Post.

The tipster said he may have seen the trio in an elevator together at the North Water Apartments, where the actor lives — and possibly after the assault occurred.

“Detectives were sent this morning to validate that tip and investigate it,” Guglielmi said. “They are investigating it. It is unconfirmed. We haven’t validated it.”

Guglielmi said investigators planned to interview the man who provided the tip.

Later Tuesday, Guglielmi said cops did not believe the tipster’s information was credible, based in part on video evidence.

Smollett claims he was attacked steps away from where he lives in the tony Streeterville ­neighborhood.

He told cops that two strangers in black clothing shouted homophobic and racist slurs — as well as “This is MAGA country!” — before dousing him with a liquid, suspected to be bleach, and looping a rope around his neck.

Smollett’s claims were widely accepted at face value and condemned by pols including presidential candidates and US Sens. Kamala Harris (D-Calif.) and Cory Booker (D-NJ), and the Rev. Al Sharpton among others, some of whom have since backtracked.

Police in Chicago have also stopped referring to Smollet as a “victim” and are instead calling him “the individual who reported the incident,” according to CBS.

Smollett has denied staging the attack, and his lawyers issued a statement Saturday that said, “As a victim of a hate crime who has cooperated with the police investigation, Jussie Smollett is angered and devastated by recent reports that the perpetrators are individuals he is familiar with.”

Police have said they want to re-interview Smollett over new evidence that “shifted the trajectory” of their investigation following interviews with the Osundairo brothers last week.

But as of Tuesday, he had still not agreed to their request. Lawyers for Smollett and the Osundairos didn’t return messages.

The buff Osundairo brothers, budding actor-models who have worked on “Empire,” considered backing out of the planned attack, sources close to the investigation told CBS’ Chicago affiliate.

The pair took a ride share near the location of the assault on North Water Street and appeared nervous in the car’s surveillance camera in the hours before it happened, sources said.

Abimbola and Olabinjo, who got there before Smollett, were also seen sitting on a bench. The actor arrived a short time later.

Two of Smollett’s six siblings posted a message of support for the actor on Tuesday — as they trashed the “irresponsible media” for its continuous reporting on the unraveling story.

With Wires