Nearly 400 people in Washington, D.C., have been convicted of driving while intoxicated based on faulty breath tests that calculated blood alcohol levels about 20 percent higher than the reality.

D.C. Attorney General Peter Nickles said the problem was caused when a police officer set improper baseline levels on the machines, the Washington Post reports. Nickles’ office is contacting the convicted drivers and their lawyers.

About 200 of those convicted spent some time in jail, the story says. At least one lawsuit has already been filed based on the revelation.