WASHINGTON -- Rep. Chaka Fattah fired back at federal prosecutors Monday, saying the officials who have brought racketeering charges against him used "unconstitutional and unlawful" threats and questionable motives to build their case.

In particular, Fattah, a Philadelphia Democrat, ripped Assistant U.S. Attorney Paul Gray, writing in a letter that Gray had told officers of the court that "earmarks were corrupt" and that Fattah was "guilty of something."

"This approach is one that is most strikingly consistent with (Department of Justice) misdeeds in multiple recent cases targeting Members of Congress," Fattah wrote in a letter to the top Republican and Democrat on the House Committee on Oversight and Government Reform.

Fattah urged the Committee to review prosecutors' conduct at the end of any trial resulting from the case, unveiled in late July. The letter comes a day before Fattah's first scheduled court appearance.

"Because this matter is pending trial, we must respectfully decline to comment," said a spokeswoman for the U.S. Attorney's office. Gray also declined to comment. A Department of Justice spokesman later wrote in an email, "the  government will respond in court to any legal challenge raised by the defendants in this case."

Fattah's five-page letter is in keeping with his aggressive public response to prosecutors' charges. For months he has alleged misconduct by investigators and has loudly touted his work for his community. He's up for re-election next year and will need to stay in voters' good graces despite the cloud of corruption charges.

Fattah's letter alleges that prosecutors have violated the rights of witnesses by approaching them without their attorneys present, broken grand jury secrecy laws in an attempt to embarrass him, used threats to coerce him into producing Congressional documents protected by federal law and of making unspecified false statements in court.

"Some in the Department of Justice may have potentially blurred the lines of the separation of powers, engaged in tactics with the intention to influence a political process, and performed other documented examples of misconduct," Fattah wrote. "This potentially unjust investigation and conduct may include elements that could ultimately mar the positive work of many well-serving staff and agents."

Fattah also cited several other cases in which members of Congress were acquitted or had their convictions thrown out after unflattering investigations harmed them politically.

He has been accused of using campaign funds, charities he created, and federal grants to prop up his failed 2007 mayoral campaign and pay family members and close political allies. Fattah has said he will be proven innocent.

Fattah's letter does not provide details of some of the accusations he makes -- such as where Gray's comments were made or the specifics of the grand jury leak.

For weeks since the charges have come out, though, Fattah -- on Twitter and elsewhere -- has taken aim at one specific piece of the indictment, in which prosecutors allege that a planned 2012 higher education conference funded in part with a $50,000 federal grant never happened. Fattah has repeatedly pointed out that a conference was held in early 2012.

But prosecutors say the grant was for an October event and that that event did not take place.

This is a developing story that has been updated with more detail and a response from prosecutors.

Staff writer Jeremy Roebuck contributed to this report.

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