Washington (CNN) -- The Department of Homeland Security has more contractors working for it than full-time employees, a situation two members of Congress said Tuesday was "unacceptable, untenable and unsustainable."

Sen. Joseph Lieberman, the chairman of the Senate Homeland Security and Governmental Affairs Committee, and ranking Republican Susan Collins said they were "astounded" to learn there are more than 200,000 contractor employees at the department.

The civilian work force of Homeland Security numbers 188,000, according to an estimate provided to the senators by Homeland Security.

In a letter sent Tuesday to the agency's Secretary Janet Napolitano, Lieberman and Collins said the figure "raises the question of whether DHS itself is in charge of its programs and policies, or whether it inappropriately has ceded core decisions to contractors."

Although Lieberman, an independent from Connecticut, and Collins of Maine noted that contractors can offer a variety of needed assets and skills to federal agencies, they called the current balance between federal employees and contractors at Homeland Security "unacceptable, untenable and unsustainable," according to the letter.

Napolitano is slated to appear before the Senate committee Wednesday and is expected to face questions on the subject.

The senators want a unit-by-unit breakdown of where in Homeland Security the contractors are working and have asked for assurances that contractors are not performing "inherently governmental work."

Clark Stevens, a spokesman for Homeland Security, told CNN Tuesday that "Secretary Napolitano has been strongly committed to decreasing the department's reliance on contractors and strengthening the federal work force" at Homeland Security.

"Over the past year, we have been actively converting contractor positions to government positions and will continue to build on these efforts at an even more aggressive pace this year. We are working across the department to identify and make additional conversions as quickly as possible while sustaining the work force required to carry out our critical mission," Stevens said.

