Convicted sexual predator Martin Tremblay has been charged in connection with the overdose deaths of two B.C. teenagers last year, police have confirmed.

Seventeen-year-old Martha Jackson Hernandez was killed by a mixture of alcohol and drugs after a party at Tremblay's home in Richmond on March 2, 2010. Her friend Kayla LaLonde, 16, was found dying of the same cause in Burnaby later that day.

After what police are describing as a "challenging" 21-month investigation into Tremblay's connection to the deaths, the 45-year-old sex offender has now been charged with two counts each of criminal negligence causing death, obstructing justice and failing to perform his legal duties to provide necessities.

"These two young girls lost their life, we're alleging, because of the actions of Mr. Martin Tremblay," RCMP Sgt. Peter Thiessen told reporters, adding that more than 100 investigators were involved in the case.

Mona Woodward, an advocate for aboriginal women, said that Martha and Kayla's deaths had a "horrendous" impact on the community.

"I know that there's tears in many people's eyes today," Woodward said.

She acknowledged that there had been a sense among many aboriginal people that the RCMP was not taking the case seriously.

"The family members are very grateful that the charges have come forward, but at the same time, it's taken a long time, so it's bittersweet," she said.

But Mounties are defending their investigation.

"It was being actively investigated since March of 2010. It was very difficult to get the appropriate evidence that we thought was needed to get charge approval," Thiessen said.

"We gained evidence every day, every week. It just took time to compile it."

Annabel Webb of the advocacy group Justice for Girls told CTV News that she was relieved to see charges finally being laid after nearly two years of uncertainty.

"We were not getting a lot of information from the RCMP. We were all worried that nothing was going on," she said.

Webb added that the Vancouver Police Department deserves a lot of the credit for its role in the investigation.

In February, Vancouver officers arrested Tremblay on drug charges as part of a sweep targeting the "worst of the worst" violent drug dealers in the Downtown Eastside. The VPD made a public plea for victims of Tremblay to come forward, promising that he was behind bars and unable to hurt anybody.

That plea led several young women to come forward and resulted in Tremblay's arrest in September on charges including sex assault and administering a noxious substance, offences he is alleged to have committed against four teenage girls. He is now awaiting trial on those counts.

Disturbing history of sex assault against aboriginal girls

Tremblay appeared in court Thursday morning to be formally charged for his alleged role in the deaths of Martha and Kayla. He has denied any involvement, and says he did not supply the girls with drugs.

But he has a long history of befriending, drugging and then raping young aboriginal girls; victims say he filmed the sex assaults.

In 2003, he was convicted on five counts of sexual assault against young girls, but was released from jail after serving just one year of his sentence without any conditions that he stay away from young girls.

Women's advocates have complained for years that not enough was done to protect young girls from Tremblay after his release from prison.

About a month after Martha and Kayla died, CTV News visited the Pender Island home where Tremblay had been living. A teenage girl answered the door, and called for "Dad."

When Martha's stepmother Connie Hernandez learned that two young girls were at the island house, she told CTV News that she was "disgusted."

"This was one of my fears, was that there was not going to be a warning put out there and that he going to continue on with being around young girls -- young, vulnerable girls," she told CTV News.

She said the RCMP should have put out a public warning about Tremblay as soon as he was released from jail.

With files from CTV British Columbia's Lisa Rossington