By David Ingram

(Reuters) - Philadelphia District Attorney Seth Williams said on Friday he would not run for reelection this year, saying he made "regrettable mistakes" when he accepted gifts that resulted in the city ethics board ordering him to pay a record fine.

Williams, a Democrat, had already been expected to face several challengers in the May primary election for the top law enforcement position in the fifth-most-populous U.S. city.

Last month, Williams agreed to pay $62,000 in civil penalties to end an investigation by the Philadelphia ethics board into his failure to disclose gifts including NBA basketball tickets, cash and lodging. The ethics board said it was the largest amount in penalties it had imposed in its 10-year history.

The gifts prompted the FBI to interview members of the prosecutor's staff and at least one of the gift-givers, and the bureau is investigating a nonprofit Williams founded, the Philadelphia Inquirer reported.

An FBI spokeswoman declined to comment on Friday.

Williams apologized at a televised news conference.

"My decisions to accept gifts and fail to report them brought much embarrassment, shame and adverse publicity to me and, unfortunately, to the office which I love," he said. "For this, I will always hold deep regret in my heart."

Another top Pennsylvania prosecutor, former state attorney general Kathleen Kane, resigned in August after she was convicted of leaking confidential grand jury information and then lying about it to investigators. Kane, a Democrat, was sentenced to 10 to 23 months in prison.

Williams, first elected in 2009 and reelected in 2013, is the first black district attorney in the history of Pennsylvania.

His 600-person office handles 40,000 criminal cases a year, from minor crimes to murders. He said he was proud of changes he had made in the office, such as involving senior prosecutors in the earliest stages of cases.

(Reporting by David Ingram in New York; Editing by David Gregorio)