The White House plans to nominate Thomas O'Brien, a career prosecutor, as the new U.S. attorney in Los Angeles, according to a Democratic Senate aide with knowledge of the decision.

The aide, who spoke on condition of anonymity because the decision was not yet public, said Tuesday that the announcement was expected this week.

O'Brien had been viewed as the leading candidate for the job. He currently is chief of the office's criminal division and before that worked in the Los Angeles County district attorney's gang division.

O'Brien would fill the vacancy created when Debra Yang resigned last October to go into private practice.

Yang was not among the eight U.S. attorneys whose firings last year have turned into a controversy for the Justice Department and has said she left of her own accord.

But Democrats had questioned the circumstances of her departure. She left while her office was investigating a powerful Republican, Rep. Jerry Lewis, R-Calif., and then she went to work for the firm representing him, Gibson, Dunn & Crutcher LLP.

Officials with the Los Angeles U.S. attorney's office and with Gibson, Dunn & Crutcher both have said that Yang recused herself properly in the matter.