Cosby's sex assault trial to begin after years of US allegations

Comedian Bill Cosby goes on trial for sexual assault on Monday in Pennsylvania, the culmination of years of allegations that have torpedoed his show business career.

Cosby, 79, once a beloved entertainer known for his family-friendly brand of comedy, is accused of drugging and sexually assaulting Andrea Constand at his home in 2004.

Constand, a former basketball player at Cosby's alma mater Temple University and more than three decades his junior, will appear as the prosecution's key witness during the trial, which is expected to last two weeks.

Her accusation is the only one to result in criminal charges out of dozens of similar allegations from women stretching back decades. Cosby has denied ever assaulting anyone, saying the encounters with Constand and others were consensual.

The trial will take place in the Philadelphia suburb of Norristown, though the jurors have been brought from Pittsburgh at the request of defense lawyers.

It was Constand's own civil lawsuit, filed in 2005 weeks after the Montgomery County district attorney's office declined to bring charges, that eventually led the same office to prosecute Cosby in 2015.

In what was then a sealed deposition, Cosby acknowledged he had obtained Quaaludes, a sedative, in order to give them to young women with whom he wanted to have sex. A federal judge released parts of the deposition in 2015, prompting prosecutors to reopen the case.

Constand has told investigators that she viewed Cosby as a mentor and that he plied her with wine and unidentified pills, leaving her unable to resist his sexual advances.

Defense lawyers will aim to undermine Constand's credibility, questioning why she failed to report the crime for a year and then could not recall certain details like the month in which it occurred.

Prosecutors will also call a second accuser, whose name is not yet public, to bolster Constand's testimony. The woman, known as Kacey, says Cosby drugged and sexually assaulted her in 1996.

(Reporting by Joseph Ax; Editing by Scott Malone and Cynthia Osterman)