NEWARK -- More than 40 years after he helped convict Harlem heroin kingpin Frank Lucas of drug-trafficking crimes, former Essex County assistant prosecutor Richie Roberts found himself in front of a federal judge Tuesday with legal troubles of his own.

Roberts, accompanied by Assistant Federal Public Defender Lisa Mack, pleaded guilty before U.S. District Judge Esther Salas in Newark to charges of failing to collect and pay payroll taxes from his Newark law firm, and also failing to pay his personal income taxes.

Speaking to NJ Advance Media by phone following the hearing, Roberts, 79, of Bloomfield, said he was "totally embarrassed."

"I've let down myself, most importantly my family, and my friends," he said. "I love the law, and I have spent my life upholding the law. To violate the law and my principles is an obscenity."

Now a private criminal defense attorney, Roberts served as a county detective and then an assistant prosecutor in the 1970s, when he obtained an indictment against Lucas -- the most powerful heroin dealer in Harlem -- for his role in a Newark drug ring.

Sentenced to 70 years in prison, Lucas became a government informant and served just six years, The Star-Ledger reported.

The case was later made into the 2007 film "American Gangster" -- starring Russell Crowe as Roberts and Denzel Washington as Lucas -- whose producers paid both men undisclosed amounts for the rights to their story.

Roberts, then in private practice, later represented Lucas during a 2011 Essex County court case involving the theft of disability assistance funds, for which Lucas was sentenced to probation.

Roberts, whose law license is suspended, failed to pay any payroll taxes for tax years 2009 through 2011, and hasn't paid personal income taxes since at least 2000, prosecutors said in a statement.

Authorities said the charges against him were the result of an investigation by the Internal Revenue Service's criminal division and investigators from the U.S. Attorney's Office.

Roberts faces a maximum potential sentence of five years in prison for failing to pay over payroll taxes, and up to one year in prison for failing to pay personal income taxes, according to prosecutors.

He also faces fines of up to $350,000, or twice his gain or the government's loss on each count.

Under the terms of his plea agreement with the U.S. Attorney's Office, prosecutors won't pursue further charges against Roberts for other tax crimes, Assistant U.S. Attorney Andrew Kogan told the court.

Salas scheduled Roberts' sentencing for Aug. 1.

Thomas Moriarty may be reached at tmoriarty@njadvancemedia.com. Follow him on Twitter at @ThomasDMoriarty. Find NJ.com on Facebook.