CHICAGO (CBS) — For the second time since Ald. Edward Burke was charged with trying to shake down the owners of a Burger King franchise, federal prosecutors are asking the judge to extend their deadline to return an indictment, citing “the complex nature of this public corruption case, and the fact that the investigation is ongoing.”

In a four-page motion filed Thursday, federal prosecutors sought a 35-day extension to return an indictment, which would give them until June 7. Typically, the feds have 30 days after a defendant is arrested or served with a summons to file an indictment, but can be granted more time if a judge decides circumstances warrant.

“The government is conducting a diligent and thorough investigation into this case, but certain factors have led to this request for an extension. These factors include the complex nature of this public corruption case, and the fact that the investigation is ongoing. Thus, due to the complex nature of this investigation, the government cannot appropriately conclude the investigation within the time allowed,” prosecutors wrote in Thursday’s motion.

The feds were granted a 90-day extension in January, nearly two weeks after Burke turned himself in on a single charge of attempted extortion.

Burke’s attorneys did not object to the first extension, and the prosecution’s motion indicates they are not objecting to this one.

Burke, 75, has been charged with one count of attempted extortion for allegedly using his position as alderman to illegally pressure the owners of a Burger King restaurant in his ward to hire his law firm for property tax work in exchange for his support for his help obtaining remodeling permits.

The criminal complaint against Burke also accuses the alderman of asking one of the executives to make a campaign contribution to another politician in exchange for his support for a restaurant renovation project.

That unnamed politician was later revealed to be Cook County Board President Toni Preckwinkle, who is running for mayor and has said she didn’t personally know about the donation or alleged extortion. Her campaign has said the entire donation was returned, and she has not been accused of a crime.

Burke resigned as chairman of the City Council Finance Committee after he was charged but was elected to a record 13th full term in office. He has been alderman of the 14th Ward since 1969, when he won a special election to fill the vacancy left by his late father, Joseph.

In late November, FBI agents raided Burke’s office at City Hall, along with his ward office in Gage Park, removing a number of computer hard drives, and several boxes of evidence. Two weeks later, they conducted a second raid of his City Hall office.