WASHINGTON — Federal regulators are failing to refer serious safety violations involving freight rail shipments of crude oil and other hazardous cargo for criminal prosecution, and are going lightly on civil fines, according to a report released Friday by a government watchdog.

The Federal Railroad Administration routinely applies only modest civil penalties for hazardous materials safety violations, even though inspectors request penalties only for serious or repeated infractions, said the report by the Department of Transportation’s inspector general.

Instead, the agency’s attorneys have made it a priority to process penalties quickly and avoid legal challenges, the report said.

And, although the agency processes hundreds of safety violations each year, it appears that not a single case has ever been referred for criminal investigation, the report said. After examining a random sample of safety violations over five years, the inspector general’s office found 17 cases it said should have referred for criminal investigation.

Based on that sample, the inspector general’s office estimated 20 percent may have warranted criminal referral. The agency’s attorneys said they didn’t make criminal referrals because they didn’t know the procedures and didn’t think it was their job.