PORTLAND — A Portland man who tried to coerce three local women into going to Boston and having sex with strangers for money was sentenced to four years in prison on Tuesday.

Samuel Gravely, 28, pleaded guilty last November to charges of interstate transportation for prostitution. U.S. District Court Judge Nancy Torresen on Tuesday sentenced Gravely to 48 months in prison, followed by three years of supervised release.

According to prosecutors with the U.S. attorney’s office, Gravely and an accomplice caused a 20-year-old Presque Isle woman and 17-year-old Portland girl to work as prostitutes at a Portland hotel, advertising the pay-for-sex services on the website backpage.com.

Gravely and his partner then allegedly picked up another 19-year-old Portland woman and transported the three victims to Boston, where they were expected to have sex with strangers.

“I think the fact that (one of the victims) was a minor makes the crime at issue much more severe, much more serious than it otherwise would have been,” Torresen said during the sentencing hearing Tuesday. “Especially if we look at the evidence in the case. We see that she was 17, on the street with just the clothes on her back and a black eye. It’s hard to escape the inference that she was chosen just because she was vulnerable.”

Another of the victims had been on her own since the age of 13, Assistant U.S. Attorney Gail Malone told Torresen.

“They were young, naive and profoundly at risk because of their circumstances,” she said.

The prostitution scheme unraveled when the third victim, who had been told they were going to Boston for shopping and sightseeing, realized what the two men had in mind and feigned sickness so she could sneak away and call the police, prosecutors say.

Gravely’s accomplice, Fritz Blanchard, 29, of Boston, was convicted for his role in the prostitution scheme late last month. Malone told Torresen on Tuesday that Gravely’s willingness to testify against Blanchard, a childhood friend, should be considered as a mitigating factor in his sentencing.

The attorney also said Gravely has been “polite” and “cooperative” with prosecutors, appears to care about the well-being of his children and completed his GED while incarcerated awaiting his sentencing.

But Malone also said Gravely’s prolonged criminal history — including previous convictions for domestic violence, drug and firearms offenses — should be looked at as aggravating factors that could increase the prison term.

“The crime that Mr. Gravely has been convicted of here is a case of manipulation and exploitation at its highest,” Malone told Torresen.

Torresen ultimately settled on a sentence of four years in prison.

Blanchard has yet to be sentenced for his role in the operation.

The convictions of Blanchard and Gravely provide a window into what police and others in Maine have warned is an expanding human trafficking scene in the state.

In late March, two men and a woman were charged in a case in which police accuse the trio of taking a 19-year-old female victim from a teen shelter and holding her against her will in Gorham, where they allegedly forced her to “earn her keep” by having sex with strangers for money.

Last July, the FBI confirmed that three of the child prostitution victims discovered during a sweeping three-day, 150-arrest international sting operation came from Maine.

Annual calls to the national Polaris Project’s human trafficking hotline from Maine more than doubled from 2009-2010 to 2011-2012, and growing awareness of the threat motivated state lawmakers to move to address the issue in recent years.

Perhaps the highest profile of those efforts was a bill by Rep. Amy Volk, R-Scarborough, to help victims of human trafficking who are forced into prostitution to cleanse their record of those prostitution crimes, among other things.