The longtime girlfriend of James “Whitey’’ Bulger pleaded guilty Wednesday to criminal contempt for refusing to testify before a federal grand jury investigating whether more people helped him during his 16 years as a fugitive.

Catherine Greig, 64, who was dressed in blue prison garb and appeared somber, briefly took the witness stand. She acknowledged under oath that she understood the charge carries no statutory maximum, meaning the judge could sentence her to as much as life in prison.

“Are you pleading guilty of your own free will because you are, in fact, guilty?’’ asked US District Judge F. Dennis Saylor.

“I am,’’ Greig said.

The judge set sentencing for April 28.

Greig, who was captured along with Bulger in June 2011 living in a rent-controlled apartment in Santa Monica, Calif., is already serving an eight-year prison term for harboring a fugitive.

Assistant US Attorney Mary B. Murrane told the judge that Greig refused to testify before a grand jury in 2014 even though she was granted immunity from prosecution for anything she revealed.

“I didn’t ask for immunity,’’ Greig told the judge.

The notorious South Boston gangster fled before his January 1995 federal racketeering indictment with another woman. A couple of months later he dropped her off and picked up Greig, a South Boston native who had a relationship with him since the mid-1970s.

Murrane said Greig refused to answer questions about her whereabouts between 1995 and 2011 “and whether any third party provided assistance to her and Mr. Bulger.’’

Greig was initially found in civil contempt for refusing to testify and nine months were added to her sentence. Prosecutors last year sought her indictment on the criminal contempt charge.

Kevin Reddington, who represents Greig, said after the hearing that his client has great respect for the court “but not so much for the government.’’

“She doesn’t understand why they are hounding her like this,’’ Reddington said.

A jury convicted Bulger in 2013 of participating in 11 murders but found him not guilty of seven other slayings.

Greig is being held at a facility in Central Falls, R.I.

Shelley Murphy can be reached at Shelley.Murphy@globe.com.