A Canadian citizen in his 30s who used to live in Arlington should spend the next decade in prison for repeatedly threatening to kill his former fiance and members of her family and for attempting to extort them from jail, federal prosecutors said Thursday.

Phillip Andrew Bauer pleaded guilty in April in federal court in Boston to charges of stalking and mailing threatening communications, records show.

In a legal filing Thursday, prosecutors asked a judge to mete out a 10-year sentence on Aug. 8. They described Bauer as “a dangerous bully who enjoys terrorizing innocent people.”

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His federal public defender could not be reached for comment.

According to an affidavit in the case, Bauer and the woman argued on Feb. 12, 2011 about their planned wedding, and he threatened to kill her if she left him and decapitate her if she contacted police, the affidavit said. He also battered the woman, prosecutors said Thursday.

She flew to California the day after the attack to live with her sister, and police arrested Bauer four days after the assault, but not before he sent several threatening texts and e-mails to the woman and relatives, according to court records.

“You will have [the woman] put my money back into my account,” Bauer wrote her to father on Feb. 16. “To lose a daughter for only 10K? ... Don’t be stupid.”

He kept up the barrage while in custody, records show.

Prosecutors said that in one letter to his former fiance dated April 23, 2011, Bauer wrote, “You and your family are going to pay for what you did to me. ... If they don’t pay me what is owed to me I am going to kill you and one of your sisters. If I can’t find you I will kill two of your sisters and let you live with that guilt.”

He continued, “I sacrificed my youth so you could go to school and become successful, now its my turn. Your parents will pay me $50,000 cash so I can attend a school of my choosing.”

Prosecutors said in Thursday’s filing that the woman has dropped out of college, relocated, changed her name, and is “extremely careful” about revealing where she lives and works out of fear that the information may reach Bauer.