By Zach Howard

CONWAY, Mass (Reuters) - A chemist at a western Massachusetts drug laboratory has been charged with tampering with drug evidence, the second such allegation at a crime lab in the state in recent months, authorities said on Sunday.

Sonja Farak, 35, was arrested late Saturday by state police and charged with two counts of tampering with evidence and two counts of drug possession, authorities said.

"We allege that this chemist tampered with evidence, placing the integrity of that evidence in question," Massachusetts Attorney General Martha Coakley said in a statement.

The charges came a month after another state crime lab chemist in Boston, Annie Dookhan, 34, was indicted on charges of falsifying drug evidence in thousands of cases. She was arrested in September.

Investigators identified some 10,000 people convicted or accused of crimes based on evidence that Dookhan handled at the Boston lab, and state officials said in December that hundreds of people had been released from prison pending new trials in the investigation.

The Amherst Drug Laboratory in western Massachusetts stores and analyzes controlled substances seized by local and state police, usually for the western half of the state.

Staff at the Amherst lab on Friday contacted State Police to report a discrepancy in the controlled substance inventory held in evidence, and police began investigating, Coakley said.

Authorities allege Farak, in one instance, removed a substance from a case that had tested positive for cocaine and replaced it with a substance that did not test positive.

Farak, who lives in Northampton, Massachusetts, was scheduled to be arraigned on Tuesday in Eastern Hampshire County District Court, Coakley said.

(Reporting by Zach Howard; Editing by David Bailey, Ellen Wulfhorst and Vicki Allen)