Two leading Democrats on the U.S. Senate committee that oversees transportation safety pressed the nation’s regulator of buses on Wednesday to provide proof it can detect troubled private companies that provide services to schools and municipalities.

The letter, from Sens. Maria Cantwell, D-Wash., and Ed Markey, D-Mass., of the Senate Committee on Commerce, Science, and Transportation, adds weight to concerns voiced by elected officials about federal regulations and Transdev. A WBUR and ProPublica investigation found that dozens of deadly collisions were missing from the safety record of the company, which is one of the nation’s largest operators of public transit.

The letter was sent to the head of the Federal Motor Carrier Safety Administration. The lawmakers asked the agency for documents showing how they ensure school bus safety data is accurate, what guidance it provides to law enforcement on how to report which organizations or companies are involved in a crash, and why FMCSA data shows the number of “enforcement actions” against companies has dropped dramatically under the Trump administration.

The senators also asked FMCSA, which is part of the Department of........

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