Saliva from a trash basket where an ex-convict spat contained DNA that matched skin cells from a T-shirt worn by a man accused of raping and torturing a Columbia University graduate student, prosecutors said.

Williams, 30, pleaded not guilty to charges that include kidnapping, arson, burglary and predatory sexual assault. He faces life in prison if convicted.

Prunty said the victim identified Williams on April 18 from a photograph as the man who had assaulted, cut, raped and tortured her for 19 hours before trying to burn her alive.

While being searched on April 20 in a room in the Manhattan district attorney's office, Williams spat into a waste basket numerous times, the prosecutor said.

"I observed defendant request to spit and observed one of the detectives, in response to defendant's request, hold a waste basket near the defendant's mouth so the defendant could spit his saliva into it," Prunty said in court papers.

After the exam, Prunty said, pieces of saliva-splattered paper from the waste basket were examined by the medical examiner's Forensic Biology Department.

An analyst in the department told Prunty that "the male DNA profile obtained from the T-shirt matches the male DNA profile obtained from the saliva sample recovered from the waste basket," the court papers said.

The prosecutor quoted a technician as saying the waste basket saliva provided a less than ideal DNA sample, so she asked for a sample directly from Williams.

Williams' lawyer, Uzamah Saghir, told Acting Justice Laura Ward the defense opposed the request for a DNA sample.

The judge reserved decision on the request and scheduled Williams' next court date for June 13.

Williams, who served eight years for attempted murder, is accused of slipping into the student's building on April 13 and forcing his way into her apartment. For hours, Williams raped the woman on her futon bed, tied her up with computer cables and used a knife to slit her eyelids, police said during the announcement of his arrest days later.

Police said he forced the student to swallow large doses of over-the-counter pain medicine as a sedative, which damaged her liver, and inflicted burns by dousing her with bleach and scalding water in an apparent attempt to destroy DNA evidence.