Had she been convicted in the United States instead of Indonesia, Heather Mack’s sentence for her role in her mother’s slaying would have been more than twice as long, federal prosecutors suggested in a memo last week.

A panel of Indonesian judges sentenced Mack to 10 years in a Bali prison for helping her former lover Tommy Schaefer plot the murder of her mother, Sheila von Wiese-Mack.

Schaefer received 18 years. Authorities say he bludgeoned the Chicago socialite to death with a fruit stand, and then, with Mack’s help, stuffed her body into a suitcase that was found outside the luxury resort where they were staying in August 2014.

The pair, who attended Oak Park-River Forest High School, would have faced more than 24 years “on the low end” in the U.S., federal prosecutors wrote in a sentencing memorandum for Robert Bibbs, Schaefer’s cousin.

Prosecutors are seeking a sentence of nine to 11 years for Bibbs, who pleaded guilty to conspiracy to commit the foreign murder of a U.S. national; he admitted to advising and encouraging Schaefer, 23, in the murder plot.

Robert Bibbs enters a court hearing in December 2016. | James Foster/For the Sun-Times

Federal murder cases are rare; under sentencing guidelines, the only options are life in prison or the death penalty.

Prosecutors likely based their estimate on a hypothetical scenario with Mack and Schaefer being tried in state court for a murder in the Chicago area, according to Jeffrey Cramer, managing director of Berkeley Research Group and a former federal prosecutor.

Mack, 21, has indicated in Cook County court filings that she is under threat of U.S. prosecution, citing “an ongoing federal criminal proceeding.” A federal magistrate judge also signed off in January on a search of an iPhone FBI investigators believe belonged to Mack.

Cramer said prosecutors probably wouldn’t try to put together a murder case against Mack or Schaefer.

“It’s more likely that they would investigate some sort of crime tangentially related to the murder, like fraud,” Cramer said.

A spokesman for the U.S. Attorney’s office in Chicago did not return messages seeking comment.

Mack gave birth to Schaefer’s daughter shortly before their sentencing in April 2015. They both could have faced death by firing squad under Indonesian law, but prosecutors sought 18 years for Schaefer and 15 for Mack.

Sheila von Wiese Mack | Sun-Times file photo

The panel of three judges acknowledged they were lenient with Mack, saying in their verdict that her “newborn baby badly needs a mother’s love and breastfeeding.”

Under Indonesian law, Mack’s daughter was allowed to stay with her in prison until she turned 2. She has since been placed with a woman in Australia and can visit her mother “anytime,” prison officials said.

Authorities have said the pair murdered von Wiese-Mack, 62, in hopes of cashing in with Mack’s multi-million dollar inheritance from her mother, and that Bibbs, 26, thought he would see some of the windfall.

Prosecutors said Bibbs “certainly could have prevented the murder had he contacted law enforcement.” In a separate memo, Bibbs’ attorney called him a “pathetic cheerleader” to the murder plot while seeking a lighter sentence.

This photo taken on Aug. 12, 2014 at a police station on the Indonesian resort island of Bali shows the suitcase in which the body of Sheila von Wiese Mack was found. The woman’s daughter, Heather Mack, along with her boyfriend, Tommy Schaefer, were later convicted in the death. They had put the body in the suitcase and it was loaded into the trunk of a taxi in front of the St. Regis hotel, where the murder occurred. | Sonny Tumbelaka/AFP/Getty Images