How exactly gangster James “Whitey” Bulger managed to elude law enforcement during his 16 years on the run is a question investigators are still trying to answer.

Authorities believe Bulger’s longtime girlfriend, Catherine Greig, may be able to help them piece that answer together. But she has refused to cooperate, remaining unflinchingly loyal to the notorious South Boston killer — even when faced with more time behind bars.

On Thursday, a judge sentenced Greig — who is already serving an eight-year sentence for assisting her boyfriend when he was a fugitive — to another 21 months behind bars for refusing to testify before a federal grand jury as it explores whether Bulger had help hiding from authorities, according to the U.S. Attorney’s Office.

Greig’s original sentence — one of the longest ever imposed for a harboring charge, according to the FBI — was scheduled to conclude in March 2019. Her new sentence will begin the same month.

Bulger — the menacing mob boss who was on the FBI’s 10 Most Wanted list — was living in a modest apartment in Santa Monica, Calif., when he was captured with Greig on June 22, 2011, following a years-long federal manhunt.

He was convicted two years later for playing a role in 11 murders during the 1970s and 1980s. The 86-year-old is currently serving two life sentences on racketeering charges, including his role in the brutal mob slayings.

In addition to extending Greig’s prison sentence, U.S. District Judge F. Dennis Saylor IV chastised the 64-year-old, calling her “openly defiant and unapologetic” and noting that she had decided to remain loyal to a “monster,” the Boston Globe reported.

“It is hard to imagine a less worthy object of affection than Bulger,” Saylor said, according to ABC affiliate WCVB.

“History, I think, will remember Bulger as a monster,” he added, noting that Greig’s loyalty was “twisted.”

With victims’ family members sitting in the courtroom, U.S. Attorney Carmen M. Ortiz said that it was Greig, and not the judge or those present, who had “control over her own fate.” Greig, Ortiz said, was forcing victims’ families to wait for justice, according to the Globe.

“Her repeated defiance translated into criminal contempt and has ultimately cost her more time in prison,” Ortiz added.

In court documents filed last week and cited by the Globe, Greig’s attorney, Kevin J. Reddington, called her “a kind, gentle woman who has literally done nothing bad in her life except fall in love with James Bulger and live with him for 16 years until their arrest.”

In 2012, Greig pleaded guilty to conspiracy to harbor a fugitive, conspiracy to commit identity fraud and identity fraud, the Globe reported. At the time, she was given an eight-year prison sentence and a $150,000 fine.

Patricia Donahue, whose husband, Michael, was fatally shot by Bulger and another man in 1982, told WCVB that Greig’s unwillingness to testify against her partner warranted even stiffer sentence than the one she received.

“How do you go before the grand jury and refuse to testify?” she said. “That’s OK to do that? That’s what they’re saying by giving her less than two years.”

Harold H. Shaw, special agent in charge of the FBI’s Boston division, said in a statement that the agency “has never wavered in its pursuit to bring Catherine Greig to justice. … Her actions adversely impacted the government’s efforts to seek answers for Bulger’s victims, and hold accountable anyone who may have helped them during their sixteen years as fugitives.”