(CNN) -- A federal grand jury has returned a sealed indictment against former New York Police Commissioner Bernard Kerik, sources familiar with the investigation said Thursday.
No details of the charges involved were immediately available. The indictment is expected to be unsealed Friday, two sources familiar with the probe told CNN.
Prosecutors had been looking into allegations against Kerik that include bribery and tax evasion, sources said earlier this week.
The indictment comes a year after admitting to misdemeanor charges of financial impropriety.
Kerik, 52, is a longtime friend and former protege of former New York City Mayor Rudy Giuliani, now the Republican presidential front-runner in national polls. Kerik was frequently by Giuliani's side in the days after the September 11, 2001, attack on the World Trade Center, and his role as the city's police commissioner propelled him into the national spotlight.
President Bush nominated Kerik to be secretary of homeland security after the president's 2004 re-election. But Kerik withdrew his name from consideration after allegations that he employed a nanny whose immigration status was questionable.
Bronx District Attorney Robert Johnson then began investigating allegations that Kerik had traded payment on repairs to his Bronx apartment for favors, including city contracts. The ex-chief pleaded guilty in 2006 to accepting tens of thousands of dollars in gifts while he worked as city corrections commissioner.
He was fined $221,000 and avoided jail time under his plea agreement.
On Thursday, during a campaign stop in Iowa, Giuliani called his earlier patronage of Kerik "a mistake." Watch Giuliani talk about the former police commissioner »
"I made a mistake in not clearing him effectively enough, and I take the responsibility for that," Giuliani told reporters at a campaign stop in Dubuque.
But Giuliani added that voters should look at his record as a prosecutor and mayor.
Referring to himself, he said, "If he makes the same balance of right decisions and incorrect decisions as president, the country would be in pretty good shape." E-mail to a friend