The retired NASA space shuttle Discovery, attached to a Boeing 747, flies over the National Air and Space Museum Steven F. Udvar-Hazy Center on April 17, 2012, in Chantilly. (Ray K. Saunders/Washington Post)

The pair of parking lot attendants at the Smithsonian Institution’s air and space center in Chantilly were very patient. And for a while, anyway, their patience paid off.

Car by car, truck by truck, SUV by SUV — as thousands of drivers handed over the $15 parking fee for the lot at the Steven F. Udvar-Hazy Center, the booth attendants pocketed it. One of them hauled off wads of cash in a red duffel bag.

To mask their thefts, they doctored the daily work sheets and unplugged the machine that counted a car every time the gate was raised and lowered.

Fifteen dollars here and $15 there added up very quickly. Since 2009, the two attendants stole almost $1.4 million in parking fees, court records show.

The full scope of the theft became known Friday at the sentencing of Freweyni Mebrahtu, 46, of Sterling, who admitted stealing $900,000. That’s on top of the nearly $487,000 theft previously disclosed during the January sentencing of her colleague, Meseret Terefe, 37, of Silver Spring.

The statement of facts filing in U.S. District Court in Alexandria.

Mebrahtu was sentenced to 27 months in prison Friday and Meseret received a 20-month term in a scheme as simplistic in its execution as it is remarkable for its yield.

The losses were about 10 percent of the total gross parking revenue for the museum in a little more than three years, according to Smithsonian officials.

Terefe stuffed the cash into his duffel bag, and then he’d toss the bag into his trunk at the end of a shift. He had $218,000 in his apartment when he was arrested last summer. He had already spent some of the rest to buy a condominium in Addis Ababa, his case files show.

Mebrahtu stole nearly $900,000 and used it to pay for her children’s college, a daughter’s surgery, cash gifts to relatives, shipments of medications and clothing to families and friends overseas and donations to mosques, according to statements Friday in U.S. District Court in Alexandria.

She consistently skimmed about a third of the fees paid at her booth and as much as $4,000 a day, court files show — or the money from an estimated 59,712 visitor vehicles.

She also schooled other attendants in how to pilfer the fees so the cash coming out of their lanes was not more than their co-workers’ and a cause for suspicion, court records show.

A confidential informant tipped federal officials to the scheme. Investigators from the Smithsonian’s Office of the Inspector General and from the FBI later painstakingly counted the vehicles passing through the parking lanes and reconciled what they counted on video and by hand with the numbers later reported by the two attendants.

Undercover cameras also led investigators to find counters being tampered with and booth attendants failing to hand drivers serialized ticket stubs, court records show.

“More than 50,000 times, Ms. Mebrahtu chose to pocket $15 in parking fees from museum visitors, amassing hundreds of thousands of dollars in a few short years,” said Neil H. MacBride, U.S. attorney for the Eastern District of Virginia. “She shared techniques with her co-workers so they could also steal money, and their crimes resulted in a huge loss to the federal government.”

The pair, who earlier had pleaded guilty to theft of public money, were employed by Parking Management Inc. The Smithsonian hired the company in 2009 to manage parking services at the museum.

The revenue sent to the Smithsonian — on a weekly basis, court files show — was always in sync with the paperwork accompanying it. Claire Brown, a Smithsonian spokeswoman, said “there were no red flags” because the annual revenue was “pretty consistent.”

The contract to PMI did not include projected parking revenue for the 2,000-space lot, so there were no signals that projections were off, Brown said.

Charles Lancaster, a company spokesman, declined to comment on the losses or procedures at the lot, citing an ongoing criminal investigation.

Both of the convicted attendants said that a supervisor skimmed part of their take as a kickback and told them to close down the scheme last July after complaints from museum visitors, court records show.

Mebrahtu’s attorney, Marvin Miller, said after the sentencing that his client “had been under pressure” and that in 2009, there had been no automated counter in the lot’s booths. The ones installed in 2010 were easily unplugged.

MacBride’s office confirmed a continuing investigation but declined to discuss its focus.

A third attendant, charged with stealing $120,000, committed suicide before her case was resolved, said MacBride’s spokesman, Peter Carr.

PMI still has the parking contract at the Udvar-Hazy Center. It will expire in April, said another Smithsonian spokeswoman, Linda St. Thomas.

The Smithsonian expects to recover some of the money through a restitution order and is pursuing negotiations with PMI. The museum also has installed tamper-proof car counters, St. Thomas said.