Cosby was arraigned Wednesday afternoon in Elkins Park, Pennsylvania, and was released after posting $1 million bail.
"A prosecutor's job is to follow the evidence wherever it leads and whenever it comes to light," Steele said. "Upon examination of all the evidence, today we are able to seek justice on behalf of the victim."
Steele said that Constand "came to consider Mr. Cosby her mentor and her friend."
He said that Cosby "made two sexual advances at her that were rejected." He added, "Mr. Cosby urged her to take pills and to drink wine, the effect of which left her unable to [rebuff his advances]."
Montgomery County Sheriff
Prosecutor say the the release of a deposition last summer from a 2005 civil suit filed by Constand, in which Cosby admitted to giving quaaludes to women he wanted to have sex with, caused the District Attorney's office to reopen the case.
Constand was not available for comment but one of her attorneys expressed gratitude toward the prosecutor.
"Obviously we appreciate the expression of confidence in her," says Dolores Troiani of her client, Constand, 42, who is now a massage therapist in Ontario, Canada. "We'll have to see what happens. We hope justice will be done. We will cooperate fully."
Cosby has previously said what happened between him and Constand was consensual, but in new court filings last summer Constand said she was in a relationship with a woman at the time. She also called Cosby a "narcissist" who missed cues that she's gay.
The statute of limitations to charge Cosby runs out in January.
Troiani exclusively told PEOPLE in September that if prosecutors decided to arrest Cosby for the alleged assault, Constand would fully cooperate.
"I think she's a very strong lady," Troiani said. "She'll do whatever she needs to do, whatever they ask of her."
Mark Blinch / Reuters / Landov
Ron Bull / Toronto Star / Zuma
So far, more than 50 women have accused the entertainer of drugging and/or sexually assaulting them. (Cosby has vehemently denied all of the allegations). The warrant comes just two weeks after Cosby filed defamation lawsuits against several of those accusers, who have filed defamation lawsuits of their own against him.
Of those lawsuits Troiani would only say, "I think this arrest warrant speaks for itself. I don't think I have to talk any louder than they have."
RELATED VIDEO: Bill Cosby Admits to Drugging Women to Have Sex with Them
Last summer, outgoing Montgomery County District Attorney Risa Vetri Ferman quietly reopened Constand's case, sending detectives to Canada to interview Constand once again while Kevin Steele, her first assistant, oversaw the investigation. (Steele is now the District Attorney-Elect.)
The existence of the new investigation only became public after the Constand case became an issue in this year's race for Montgomery County district attorney.
Democrat Steele, who ultimately won, ran a TV ad blasting his opponent, Republican Bruce L. Castor, Jr. for not prosecuting Cosby when he was the county's district attorney in 2005. At the time, Castor cited "insufficient credible and admissible evidence."
Castor responded with an ad of his own criticizing Steele, the county's first assistant district attorney, for not taking action himself.
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In late October Constand filed a defamation lawsuit against Castor for comments he made to various media in the preceding weeks, saying he had made her "collateral damage for his political ambitions," according to the complaint.
How It All BeganConstand was operations manager of the Temple University women's basketball team when she first met Cosby in November 2002, about a year after she started working there, according to a civil suit she filed against him after Castor initially decided not to prosecute him. (Cosby settled the lawsuit with a confidentiality agreement, which Constand is trying to undo, in late 2006).
Cosby, who is now 78, "fostered a friendship" with her, "so that over time she considered him to be both her friend, albeit older, and a mentor," the suit said.
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In January 2004, Cosby invited Constand to his mansion, "telling her he wanted to offer her assistance in her pursuit of a different career," the suit said. She agreed to meet him and arrived at his home around 9 p.m. They began talking and, during the conversation, Constand said she was feeling "stressed" about making her career decision, the suit said.
Cosby "then offered [her] three blue pills, which he told her were herbal medication, which would help her relax," the suit said. Constand took the pills with bottled water and within a short period of time, her "knees began to shake, her limbs felt immobile, she felt dizzy and weak, and she began to feel only barely conscious," the lawsuit said. (Cosby had denied drugging Constand, saying that he gave her Benadryl.)
She allegedly told Cosby she didn't feel well and, because she couldn't walk on her own, he led her to a sofa and laid her down, the suit said.
"Subsequently, [Cosby] positioned himself behind [Constand] on the sofa, touched her breasts and vaginal area, rubbed his penis against her hand, and sexually assaulted her," the suit alleges. Constand was "semi-conscious" throughout the ordeal, the suit said, then lost consciousness shortly afterward.
She allegedly woke around 4 a.m., "feeling raw in and around her vaginal area," the lawsuit said. Her "clothes and undergarments were in disarray," the suit said.
After she woke up, Cosby greeted her "in his bathrobe," the suit said. She left his home by herself and, one year later, reported his actions to police in Durham, Ontario then was referred to authorities in Montgomery County, Pennsylvania, where the alleged incident occurred.