Actor Jussie Smollett, centre, leaves the Leighton Criminal Courthouse in Chicago after prosecutors dropped all charges against him. Picture: Ashlee Rezin/Sun-Times/Chicago Sun-Times via AP

Chicago - The FBI was already investigating Jussie Smollett before President Donald Trump tweeted that the agency and the Department of Justice would review the case, which the president called "an embarrassment" to the nation.

Trump's tweet on Thursday came two days after a surprise decision by Cook County prosecutors to

Some former federal prosecutors say there are enough anomalies to justify a Justice Department review of why prosecutors dismissed all 16 felonies. Among the red flags they cited: the unusual level of secrecy around the proceedings, including sealed court documents; prosecutors' often muddled and contradictory explanations; and the absence of any requirement that the accused accept responsibility in exchange for dropping charges.

Investigators believe Smollett hired two brothers to stage the January 29 attack and that Smollett hoped the attention would help advance his career. Police also allege that before the attack, Smollett sent a letter threatening himself to the Chicago television studio where "Empire" is shot. The FBI has been investigating that letter.

State prosecutors insisted that they could still prove Smollett concocted the entire assault. And even after charges were thrown out, the actor maintained that the attack was real.

Cook County State's Attorney Kim Foxx has defended the decision by her staff to drop charges that "Empire" actor Jussie Smollett staged a racist, anti-gay attack in January. Foxx recused herself before Smollett was charged last month because she had discussed the case with a Smollett family member. Picture: Kiichiro Sato/AP

Phil Turner, an assistant US attorney in Chicago before entering private practice, said federal prosecutors could, at a minimum, seek an order for state prosecutors to preserve emails, texts and other documents that might shed light on why they acted as they did with Smollett.

Prosecutors' hasty calling of the Tuesday hearing with no prior public notice and their swift sealing of court records showed a highly unusual and suspicious level of secrecy, Turner said.

"It makes it look like they were clearly trying to pull a fast one," he said. "It all smells funny."

Federal prosecutors could, in theory, bring charges under a law that requires public officials to provide honest services to citizens if evidence emerges that any decision was made at the behest of influential people outside the state's attorney's office, Turner said.

Justice Department investigators "could try to show that, if Cook County prosecutors did a favour for someone by dropping charges, that deprived people in Cook County of the honest services they deserve," he said.

Actor Jussie Smollett, right, stands with his attorney Patrica Brown Holmes during a news conference at Cook County Court after Smollett's charges were dropped. Picture: Paul Beaty/AP

The case, he theorized, could qualify as a federal matter if communications about the decision took place on the phone or internet, placing it within federal jurisdiction.

Another former federal prosecutor, Joel Bertocchi, sounded much more sceptical about the chances of a full-fledged investigation.

"You'd have to show the decision (to drop charges) was corrupt — not just bad or unexplainable," he said. "Traded favours is not enough for corruption. It has to be money. And there's no indication of that here."

While US attorneys can in principle look into whatever they want, Bertocchi said, the accepted practice is that they need some indication an actual crime was committed. "You can't just investigate someone willy-nilly."

The suspicions about the Smollett charges were shared by many in Chicago, including Mayor Rahm Emanuel, who has led official voices of outrage, telling reporters that "something is rotten in Denmark." The city said Thursday it is seeking $130 000 from Smollett to cover the costs of the investigation into his reported beating.

The Illinois Prosecutors Bar Association on Thursday issued a scathing critique of Cook County prosecutors. The group said State's Attorney Kim Foxx and her representatives "fundamentally misled the public on the law and circumstances surrounding the dismissal."

Chicago Mayor Rahm Emanuel, right, and Chicago Police Superintendent Eddie Johnson appear at a news conference after prosecutors abruptly dropped all charges against "Empire" actor Jussie Smollett, abandoning the case barely five weeks after he was accused of lying to police about being the target of a racist, anti-gay attack. Picture: Mitch Armentrout/Chicago Sun-Times via AP

"The manner in which this case was dismissed was abnormal and unfamiliar to those who practice law in criminal courthouses across the State," it said.

The critique also highlighted what it called the "uncontested sealing" of the case, saying Cook County prosecutors had "falsely informed the public" that the action was mandatory under Illinois law.

The fact that Tuesday's hearing was not put on the public court schedule and other factors added to an "appearance of impropriety," according to the bar association, which said Cook County prosecutors had "fallen woefully short" of legal ethics.

The Arlington, Virginia-based National District Attorneys Association also weighed in, saying in a statement that aspects of the case may have run afoul of best practices, including the decision to abandon the charges without forcing Smollett to accept some degree of guilt.

The group alluded to Foxx's decision to recuse herself before Smollett was charged because she had discussed the case with a Smollett family member. If the chief prosecutor withdraws, it said, the best practice is for the whole office to step away from a case and to appoint a special prosecutor.

Foxx defended her office's decisions. Even if a trial had been held, Smollett would never have received a prison term because the 16 charges were the lowest form of felony. And if he pleaded guilty, it would have been to one count, not 16, she said.

"For all of the noise and all of the media attention, it's a step up from a misdemeanour," she told Chicago television station WGN. Most comparable cases are resolved with probation or an agreement such as Smollett's if the defendants "don't have a long rap sheet or violence in their background."

AP