But in a defiant, nine-minute press conference afterwards, Grimm vowed to prove his innocence in court. He portrayed himself as the victim of a “vendetta” and “political witch hunt” by his opponents.

“I know who I am, and I know what I’ve done for this country… I know I’m a moral man, a man of integrity,” Grimm declared. “We’re going to fight tooth and nail until I am fully exonerated.”

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“Time and time again I have shown I don’t abandon my post, Grimm added. “I didn’t abandon my post when I was being fired upon in combat,” or when he was an FBI agent. “I’m not abandoning my post now.”

The allegations against Grimm, 44, are related to his ownership of a health food restaurant in Manhattan called Healthalicious. Grimm owned the restaurant after he left the FBI in 2006 until he was elected to the House in 2010.

The New York Republican was taken into custody prior to his arraignment on the federal charges, which took place early Monday afternoon, said Loretta Lynch, the U.S. Attorney for the Eastern District of New York. Lynch’s office oversaw the Grimm investigation, which initially focused on campaign fundraising from his 2010 House race but then shifted to his pre-congressional activities as a restaurant owner.

( DRIVING THE DAY: What’s next for Michael Grimm?)

Grimm, quiet and seemingly a bit unsure of his footing as he was led by an FBI agent into the Brooklyn courtroom, was released on a $400,000 bond, using his home as collateral. Grimm will be represented in the case by John and Elizabeth Kase, two New York City defense attorneys. Grimm said little other than “Yes, your honor” during the proceedings before Magistrate Judge Robert Levy.

( Also on POLITICO: Grimm could cost GOP swing seat)

Yet the indictment unveiled by Lynch lays out a long list of criminal charges against Grimm, including mail and wire fraud, filing false tax returns, health care fraud, hiring undocumented workers, perjury and obstruction.

Lynch held a scathing press conference denouncing the one-time FBI agent and Marine.

“In 2007, Michael Grimm - a former Marine, former FBI agent who knew what fraud looked like, accountant, and attorney - was poised for success as a small business owner,” Lynch said at her press conference in Brooklyn. “Instead, as alleged, Grimm made the choice on upholding the law to breaking it. In doing so, he turned his back on every oath he ever took.”

Lynch and her prosecution team allege that Grimm failed to report more than $1 million in income, showing his restaurant - Healthalicious - operating at a loss rather than a profit.

( QUIZ: How well do you know Michael Grimm?)

Lynch stated that “more than $1 million simply disappeared” from the restaurant’s books thanks to Grimm’s action. Grimm reportedly had a “systematic plan to deliberately lie” to every tax-reporting agency. After Grimm stepped out of the day-to-day management of the restaurant, he became a “fraud instructor” showing other how to cheat the system, Lynch asserted.

“Even after his return to public service, when called to account for his actions and questioned under oath, Grimm went for the cover up, and lied about his role in his own business,” said George Venizelos, the assistant director in charge of the FBI’s New York office. “As a former FBI agent, Rep. Grimm should understand the motto: fidelity, bravery, and integrity. Yet he broke our credo at nearly every turn.”

Grimm lived by “a new motto: fraud, perjury and obstruction. We demand the best from our political leaders. Yet today we again find ourselves wanting more,” Venizelos said.

Grimm’s New York-based defense attorney did not respond to repeated calls or emails on Monday. Sources close to Grimm expect him to hold a press conference to discuss the allegations.

GOP leaders on Capitol Hill, including Speaker John Boehner, haven’t yet commented on Grimm’s situation.

According to the 30-page indictment, Grimm “engaged in schemes to fraudulently under-report the wages he paid his workers - many of whom did not have legal status in the United States - and fraudulently under-report the true amount of money the restaurant earned to both federal and New York State tax and insurance authorities.”

Grimm allegedly paid the restaurant employees partially in cash, and then didn’t report those earnings to either the IRS or New York State, the indictment asserts.

“In total, Grimm concealed over $1,000,000 in Healthalicious gross receipts alone, as well as hundreds of thousands of dollars of employees’ wages, fraudulently depriving the federal and New York State governments of sales, income and payroll taxes,” said the indictment.

When questioned about Healthalicious’ operations in 2013 as part of a civil lawsuit brought by two former restaurant employees - a full three years after he won his House seat - Grimm allegedly continued to lie about it, the indictment states.

Lynch said Grimm’s coverup was sparked by that lawsuit and his “deception continued into 2013.”

The federal prosecutor, appointed by President Barack Obama to her post in 2010, said the Grimm probe is continuing, and sources close to the case said a superseding indictment was possible. Lynch would not comment on whether her office has had contact with and or interviewed Grimm.

Lynch, though, rejected claims of playing politics by indicting Grimm after the filing deadline closed for his race. Grimm is now almost certain to be on the ballot in November, giving Democrats a pick-up opportunity on a seat they’ve long targeted.

“When you investigate a case like this, we follow the evidence,” Lynch said. “It’s just as irresponsible to bring it before it’s ready.”

Lynch said the various counts carry maximum sentences of up to 20 years, while others carry potential five and 10-year terms.

She said Grimm is just another example of a politician who “let their greed in their personal life implicate their actions even after they’ve taken office.”