Black Hawk helicopters and active-duty soldiers are part of a secret ongoing military operation being conducted around the nation’s capital.

Existence of the operation was revealed as part of a funding request to Congress, according to Bloomberg. The Defense Department asked Congress for approval to shift an extra $1.55 million in funds toward air crews, aircraft maintenance, and travel as part of an “emerging classified flight mission.”

The funding shift was part of a larger $2.5 billion request to “reprogram” money from the budget to operations and programs considered top priorities. In the document, the Defense Department mentions the Black Hawks being used in the mysterious operation.

“Soldiers from assault helicopter company and aviation maintenance units will be supporting the mission with 10 UH-60s and maintenance capabilities for four months,” it added, also noting that the funds would be used for a “Sensitive Compartmented Information Facility” at Davison Army Airfield at Fort Belvoir, Virginia. The fort is located just south of Washington, D.C., near Alexandria, Virginia.

“Without additional funding, the Army will not be able to perform this classified mission,” the Defense Department said.

Few other details are available about the operation, which began Oct. 1 and has an undetermined end date.