Seven US senators on Thursday called on the Pentagon to formally investigate a document mix-up in the politically-charged contest for a new aerial refueling tanker.

With a decision on the contract expected in February or March, the senators demanded the Defense Department’s inspector general probe whether EADS gained an unfair advantage when the Air Force inadvertently sent the company and its rival Boeing each other’s data.

The lawmakers issuing the call come mainly from states that stand to benefit if Boeing wins the $35 billion contest against European aerospace giant EADS, parent of Airbus.

“From the beginning, we believe the Air Force has attempted to minimize its mistake,” said a letter sent to the Pentagon and signed by the senators.

The European Aeronautic Defence and Space Company and Chicago-based Boeing are vying to build a new fleet of mid-air refueling tankers to replace the Air Force’s ageing fleet, which dates back to the 1950s.

The contest marks the Pentagon’s third attempt in a decade to secure a contract for the planes and the Air Force’s “clerical error” in November has fueled an intense battle in Congress over the deal.

The Senate Armed Services Committee convened a hearing on Thursday focused solely on the document mix-up, with senators allied with each company clashing over the impact of the mistake.

“This is a case study of incompetence in contract competition, this whole debacle, from beginning to this very moment,” Senator Clair McCaskill said.

In the Air Force foul-up, employees at the rival firms loaded a disc with their competitor’s technical information.

An EADS worker admitted to briefly opening a screen display showing Boeing’s information before closing it while Boeing maintains its employee never opened any files.

At the hearing, McCaskill and other senators grilled a Pentagon computer expert about how long the file remained open on the EADS computer.

The expert, Steven Shirley, executive director of the Pentagon’s cyber crime center, said an investigation showed the EADS employee had the file open for about three minutes and that the computer remained on for 20 minutes.

EADS says the employee only viewed the file for 15 seconds before leaving the room and phoning a colleague to get help closing and isolating the file under company procedures, Shirley said.

After the incident, the Air Force released the data to both competitors to ensure each had the same information. The firms also were allowed to change their bids afterward if they wanted.

Pentagon press secretary Geoff Morrell said on Wednesday the incident was not expected to in any way derail the tanker competition.

The effort to launch a new fleet of refueling aircraft has been marred by scandal and bitter feuds.

Two previous attempts to move ahead with a new tanker were canceled, first with Boeing and then with EADS and its US partner Northrop Grumman.