Staten Island Republican U.S. Rep Michael Grimm hired undocumented workers at a restaurant he owned before his election, avoided paying taxes on their wages and lied about it under oath, according to a 20-count indictment unsealed in Brooklyn on Monday.

Grimm, who was elected in 2010, started the "Healthalicious" restaurant in 2007, shortly after leaving the FBI. He was the target of a well-publicized investigation of campaign finance irregularities for two years, but was accused only of the restaurant scheme.

"He chose to violate every oath he had ever taken," said Brooklyn U.S. Attorney Loretta Lynch, who said the investigation was ongoing and "broader" charges were still possible. "When it came to his business, Michael Grimm never met a tax he didn't lie to evade."

Grimm, 44, surrendered Monday morning, and was released after pleading not guilty. In an appearance at a veterans' memorial a block from the federal court in Brooklyn, he said he would stand for re-election in November, but declined to comment on the charges.

"We're going to fight tooth and nail until I'm fully exonerated," said the ex-Marine, who accused prosecutors of pursuing a "political witch hunt."

According to the indictment, Grimm opened his health-food restaurant on Manhattan's Upper East Side in 2007 at the end of an FBI career that had included a stint as an undercover agent investigating fraud on Wall Street.

Prosecutors charged him with knowingly hiring workers illegally in the United States, paying them with cash, falsifying his payroll records underreporting the gross receipts of the business by $1 million, cheating on sales, income and payroll taxes from 2007 to 2010.

He also underpaid workers' compensation premiums, the indictment said, and after his election to Congress in 2010 lied under oath in a deposition in a civil suit brought by ex-workers alleging minimum wage and overtime pay violations.

He faces up to 20 years in prison on each of the top charges in the indictment, but House rules do not prevent him from continuing to serve or running for re-election while the charges are pending.

The indictment was unsealed just after the deadline for Republicans potentially putting a different candidate up for his seat, but Lynch said there was no political motive behind the timing of the indictment. "We bring it when it is ready," she said.

After the Senate approved a comprehensive immigration bill last June, Grimm expressed support for implementing an "E-Verify" system to crack down on illegal workers by allowing employers to check the eligibility of prospective employees.

In a statement to the Staten Island Advance he said he backed a plan that "closes our border, mandates E-Verify so that we don't have illegal workers, and fixes our broken visa system."

Grimm is due back in court on May 19 for a pretrial hearing.

With Tom Brune.