Car-by-car over the past three years, a parking lot attendant at the Smithsonian Institution’s Steven F. Udvar-Hazy Center in Chantilly dutifully collected a $15-a-vehicle fee — and then routinely stashed wads of the cash into a duffel bag and took it home with him, court records show.

Wads as in: $400,000 stolen between March 2009 and July 2012, according to the guilty plea entered Friday in federal court in Alexandria by attendant Meseret Terefe, 36, of Silver Spring. Terefe usually worked only two days a week — often weekends — yet could steal as much as $4,470 a day by under-reporting cars that had paid, court filings show.

During seven days beginning in late May, about $23,000 in parking revenue was not reported when Terefe failed to count hundreds of cars that passed through his booth — one of five entry lanes at the lot, court files show.

Terefe, according to his plea, would disconnect the devices that counted vehicles as they passed through entry gates and would fail to hand drivers serialized ticket stubs used to track tallies. Undercover cameras helped document those diversions, court records show.

Terefe was brought into the stealing scheme by a co-worker in 2009 who pulled him aside to say that because he was not stealing, his then-high counts of cars “was putting others, including her, at risk,” as they pilfered parking fees, according to admissions Terefe made as part of his plea in United States District Court for the Eastern District of Virginia. In time, his plea shows, a manager also approached Terefe asking for half of what Terefe was stealing — and Terefe paid that, court records show.

Terefe was arrested in August. The federal case is ongoing, prosecutors said. Terefe’s plea refers to other employees only by initials.

The 2,000-slot parking has been managed since March 2009 by Parking Management, Inc., court files show.

PMI President Kingdon Gould III on Friday said he did not know the details of Terefe’s plea. But, Gould said, the company had alerted federal officials about apparent losses “and we have been cooperating in this investigation since then and continue to cooperate to help them get the bad guys.”

The incident, Gould said, “was an all around miserable thing” for his company.

Terefe stored some of the stolen cash at his home and used some to buy an interest in commercial property in Ethi­o­pia, prosecutors said. He faces sentencing on Dec. 14.

The case was initiated by the Smithsonian Office of the Inspector General and jointly investigated by the FBI’s Washington Field Office.