SAN FRANCISCO — The tape recorder started rolling as two police investigators sat in their car in a hospital parking lot with Deborah Madden on Feb. 26. "You're causing a huge nightmare for the city," said one officer. Now the 60-year-old technician and the obscure police crime lab where she worked for 29 years stand at the center of a scandal that has led to the dismissal of hundreds of criminal cases and jeopardized thousands more. Forensics experts say Madden is not the first crime lab worker suspected of stealing drugs or other illegality, and San Francisco's lab joins several other cities in suffering a loss of credibility. A Houston man was awarded $5 million last year after spending 17 years in prison on rape charges overturned because of a discredited criminal lab. Detroit shut down its crime lab in 2008 after outside auditors uncovered serious errors in the way evidence was handled. "It's real hard to build a good reputation and it's very easy to destroy it," said Ralph Keaton, executive director of the American Society of Crime Laboratory Directors.