The criminal charges stem from a woman’s accusation that he drugged and sexually abused her at his home in a suburb north of Philadelphia in 2004.
Kevin Steele, the Montgomery County district attorney-elect, said that Mr. Cosby faces felony charges of indecent assault. He said the investigation involved a “relationship” between Mr. Cosby and the woman, Andrea Constand, that came about from her work with the basketball team at Temple University, Mr. Cosby’s alma mater.
“The evidence is strong and sufficient to proceed,” said Mr. Steele, now first assistant district attorney. “A person in that state cannot give consent,” he added.
The criminal charges followed a year of tumult in Mr. Cosby’s career as dozens of women accused him of sexually assaulting them. Many of those women cheered the decision to prosecute Mr. Cosby, the entertainer who was once revered as a father figure and popular moralist.
Lawyers for Mr. Cosby, 78, who has long denied the many accusations and had never before been criminally charged, had only recently spoken of how he had been harmed as universities and Disney World retracted honors they had bestowed on him.
The lawyers said they would fight the charges.
“Make no mistake, we intend to mount a vigorous defense against this unjustified charge and we expect that Mr. Cosby will be exonerated by a court of law,” Monique Pressley, one of his lawyers, said in a statement.
Document: 2015 Criminal Complaint Against Bill Cosby
Mr. Steele said the decade-old case had been reopened after new evidence emerged in July. He referred to court documents and the deposition from a civil suit filed by Ms. Constand in which Mr. Cosby acknowledged he had given women quaaludes as a party drug in his efforts to have sex. Some court records were sealed when the civil case was settled in 2006 but were released by a federal judge in July.
In the criminal complaint filed Wednesday, investigators said one of the most important factors to them was Mr. Cosby’s “conflicting identifications of the drug” he gave Ms. Constand. She said he described the drugs as herbal, and Mr. Cosby told the police the drug was Benadryl but told Ms. Constand’s mother he could not recall what it was.
Mr. Cosby’s apologies to Ms. Constand and her mother and offer of financial assistance was “further indicative of Cosby’s consciousness of guilt,” according to the complaint.
“Investigators recognize that individuals who are falsely accused of sexual assault generally do not unilaterally offer generous financial assistance, and apologies, to their accuser and their accuser’s family,” the complaint says.
A predecessor at the district attorney’s office, Bruce Castor, declined to prosecute Mr. Cosby in the case in 2005, saying he did not believe there was sufficient evidence. That decision was an issue in the fall race for Montgomery County district attorney, between Mr. Castor and Mr. Steele.