WASHINGTON (AP) — Federal regulators are failing to refer serious safety violations involving freight rail shipments of crude oil and other hazardous cargo for criminal prosecution, according to a report Friday by a government watchdog.

The Federal Railroad Administration routinely applies moderate penalties no more how serious the safety violation in order to avoid litigation, said the report by the Department of Transportation's inspector general. A random sample of safety violations over five years found 17 cases that agency should have referred for criminal investigation, the report said.

The agency also doesn't have a complete understanding of the risks of rail shipments of hazardous cargo, including more than 400,000 tank cars of oil shipped across the country annually, the watchdog found. That's because agency only looks narrowly at operations in specific regions, not the nation as a whole, the report said.

The regional evaluations also don't include an assessment of the risks of transporting highly volatile and hazardous materials like crude oil near cities and major population centers, the report said.

There has been a series of fiery oil train explosions in the U.S. and Canada in recent years, including one just across the border in Lac-Megantic, Quebec, that killed 47 people.

The inspector general also faulted the agency's complex records system, saying it makes difficult for inspectors to access safety information on rail operations outside their region. As a result, the railroad administration and a sister agency, the Pipeline and Hazardous Materials Safety Administration, don't share critical and up-to-date information with safety inspectors and investigators in different regions throughout the country.

The report confirms "that the federal government has failed to provide the necessary oversight to protect communities across the country from serious accidents involving the rail transportation of hazardous materials," said Rep. Peter DeFazio, D-Ore.

Matt Lehner, an FRA spokesman, said most of the inspector general's recommendations are being implemented. He noted that the agency collected $15 million in fines for violations in the 2015 federal budget year, a 12 percent increase over the previous year and the most in the agency's history.