The detective who led the investigation that helped put convicted killer William Shrubsall behind bars for violent sexual assaults against women in Halifax is criticizing the decision to deport him to the U.S.

"He is a predator when it comes to women. And the parole board doing this, it's disgusting and it's borderline negligence as far as I'm concerned," Tom Martin told CBC News.

Now that he's back in the U.S., Shrubsall, 47, could be eligible for parole in less than five years.

He was declared a dangerous offender in 2001, and if he had not been released from prison in Canada, he might have stayed behind bars for the rest of his life.

A law enforcement officer leads William Shrubsall through the Niagara County Court House in Lockport, N.Y., on Tuesday. (Tim Fenster/The Union-Sun & Journal via AP)

​Martin says he spoke to three of the four women in Nova Scotia who survived Shrubsall's violent attacks and said they're confused and angry. He said the decision to release him is not fair and "just not right."

"What those women went through with him and then what they went through with the court process and then … the parole board — who answers to apparently nobody — just let him go."

While he was in prison, Shrubsall changed his name to Ethan Simon Templar MacLeod. He is studying seminary courses.

Martin said he thinks Shrubsall is "just saying what he thinks people want to hear."

"That's how this guy operates. He is very manipulative," Martin said.

Shrubsall committed his first violent crime in his teens. He was convicted of manslaughter for beating his own mother to death.

He ended up in Canada after he faked his own death while he was on trial for sexual assault in Niagara Falls, N.Y.

Shrubsall was convicted in absentia and a seven-year prison sentence awaits him upon his return.

On Tuesday, the district attorney for Niagara County told The Canadian Press she "can't think of anyone better to confine for the rest of his life."

The Parole Board of Canada told CBC News this week it cannot discuss specifics of Shrubsall's case — a comment that Martin took exception to.

"They're a public board. This has gone through court. Why can't they discuss it? Why is it closed? Why are they not releasing the minutes or the recordings of the actual hearing itself?" Martin said.

"Why would they do that? That, in and of itself, a public entity, is very concerning to me."