CLAYTON • A crime lab scientist has resigned after police say she did not properly test for marijuana in drug cases.

The scientist had worked for the lab for more than 20 years, but police are confident she only skipped proper testing procedures during the month of September. Her name will not be released until the internal affairs investigation is complete, said St. Louis County Police Chief Tim Fitch.

"We're confident that we've narrowed it to only seven cases," Fitch said, adding that investigators still are trying to determine the employee's motive.

The woman's coworkers tipped off a supervisor that she had been taking shortcuts on testing procedures. She had been suspended with pay on Friday, but resigned today, Fitch said.

"She submitted lab reports saying she tested it when in fact she didn't," Fitch said.

A different scientist conducted new tests in the seven cases and confirmed the tested substances are marijuana, Fitch said.

Whether the woman will be criminally charged is up to St. Louis Prosecutor Bob McCulloch.

"The good thing is that it was caught by our quality assurance procedures," Fitch said. "I credit the other scientists' ethics."

Fitch said the St. Louis County police will be hiring an independent auditor to review the employee's work. That could take about 60 days to complete.

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EDITOR'S NOTE: In an earlier version of this story, police gave a possible motive for the employee's alleged misconduct. Authorities now say the reason is under investigation.