The chemist at the center of a scandal at a Massachusetts drug lab admitted to investigators that she faked test results for two to three years, forged signatures and bypassed proper procedures, according to a state police report obtained Wednesday by The Associated Press.

The report indicates that Annie Dookhan told police several times that she knew what she had done was wrong.

“I screwed up big time,” she said, according to the report by investigators for Attorney General Martha Coakley’s office. “I messed up bad, it’s my fault. I don’t want the lab to get in trouble.”

Dookhan’s alleged mishandling of drug samples at the lab has thrown thousands of criminal cases into question, and a handful of defendants already have been freed or had their sentences suspended.

Dookhan has not been charged and investigators have not said what her motive was. She has not responded to repeated requests for comments from the AP and no one answered the door at her home in Franklin on Tuesday.

State police say Dookhan tested more than 60,000 drug samples submitted in the cases of about 34,000 defendants during her nine years at the lab. She resigned in March amid an internal investigation by the Department of Public Health.

After state police took over the lab in July as part of a state budget directive, they said they discovered her alleged violations were much more extensive than previously believed and went beyond sloppiness into malfeasance and deliberate mishandling of drug samples.

In an interview with investigators in August, Dookhan first denied doing anything wrong when analyzing drug samples, then changed her story when they confronted her with a cocaine sample that tested positive in her analysis but came back negative when retested by the Boston Police Department.

She admitted doing something called dry labbing, where she tested some samples properly but just looked at others and guessed that they were the same drug as the properly tested ones. If a second test called the results into question, she would tamper with the drugs by concentrating them or contaminating them with other drugs so it did not look like she had improperly labeled them.

“Dookhan explained that this was what she did to get more work done,” investigators wrote in their report.

Dookhan also told investigators she routinely skirted proper procedures by looking up data for assistant district attorneys who called her directly rather than going through the evidence department. She says none ever asked her to do anything improper in her analysis or findings.

The report, first obtained by The Boston Globe, says Dookhan told an investigator she was going through a long divorce and had no money for an attorney.