Sen. Barbara A. Mikulski, at a Maryland Democratic Party reception this year. (Bill O’Leary/The Washington Post)

Sen. Barbara A. Mikulski (D-Md.) has secured $390 million for a new FBI headquarters as part of the spending agreement that was negotiated by Democrats and Republicans in Congress.

The agency is in the first stages of relocating from the aging J. Edgar Hoover Building downtown to a new suburban campus.

The search for a new home began three years ago, and three potential sites have been chosen — two in Maryland and one in Virginia.

Mikulski and her fellow Maryland lawmakers have been pushing vociferously for their state to land the headquarters and the 11,000 workers it would hold. State officials have promised to invest millions in road improvements to bring the FBI to Greenbelt or Landover in Prince George’s County, rather than to Springfield, Va.

The money secured in the budget negotiations, however, would be available regardless of where the new headquarters is built.

When her party held the majority in the Senate, Mikulski was chair of the powerful Appropriations Committee. Once Republicans assumed the majority, Mikulski lost that role.

But the funding announcement suggests that even in the minority, Mikulski continues to wield considerable influence.

“I’ve fought tooth and nail for the men and women of the FBI, just as they fight each day to keep our communities safe from criminals and terrorists,” Mikulski said in a statement. “This is a headquarters that belongs in Prince George’s County.”

The language was not in the initial spending bills proposed by the House and Senate; it came together in joint negotiations between the two chambers.

The General Services Administration plans to cover much of the cost of a new building by swapping the valuable current site for a new one on less expensive land, thus avoiding the need for congressional appropriations. But most private-sector experts think there will be a gap in funds to pay for the new building, and uncertainty about funding hovers over the project.

Mikulski’s language includes $180 million for the FBI in new construction funds, $135 million from funds not spent in prior years for land acquisition and other pre-construction work, and $75 million for the GSA for management and oversight.

Mikulski described the money as a “down payment” for whatever appropriations are needed. The deal must be voted on by the House and Senate.

An environmental assessment of the three potential FBI sites is close to completion. A draft has been released.

In October, the federal government confirmed that the site would be a consolidated one, bringing together the employees of 14 sites across the region.