(CNN) A key official at the agency responsible for overseeing the nation's railroad safety has resigned after "troubling" allegations that he was still working as a public relations consultant in Mississippi.

Heath Hall, who was appointed deputy administrator of the Federal Railroad Administration last June and later named the acting administrator, "has resigned his position at the Department effective immediately," a Department of Transportation spokeswoman told CNN Saturday.

"DOT was unaware of the information that is being reported regarding outside work Heath Hall took on during his time at FRA, but those allegations, if true, are troubling," the DOT spokeswoman told CNN.

While serving at the Federal Railroad Administration, Hall was also quoted in at least one Mississippi news article as a spokesman for the Madison County sheriff, which was a client of the public relations and consulting agency he owned in Mississippi.

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