A trio of Virginia legislators is asking the U.S. Department of Justice to investigate the state’s use of solitary confinement in prisons, especially of those who are mentally ill.

Dels. Patrick A. Hope (D-Arlington) and Charniele Herring (D-Alexandria) and Sen. Adam Ebbin (D-Arlington) wrote a letter to Attorney General Eric Holder this week asking for the inquiry after attempts to secure a study failed in the General Assembly this session.

“I believe Virginia has been engaged in a form of psychological torture of certain prisoners in segregation,’’ Hope said. “Much of it is unnecessary and all of it is because we don’t know there is a more effective and humane way to house prisoners.”

A spokeswoman for the Justice Department did not immediately respond to the letter. Larry Traylor, a spokesman for the state Department of Corrections, said the agency is disappointed by the letter. He said that mentally ill prisoners are “not isolated or in solitary, they are segregated,’’ which means they may communicate with other offenders, phone friends and family and have visitors. “To equate segregation with isolation is simply incorrect,’’ he said.

The Northern Virginia legislators introduced the bill after visiting state prisons this fall, including Red Onion in Southwest Virginia, to examine how their most violent inmates are treated.

Virginia, one of 44 states that uses solitary confinement, had 1,731 people in isolation, about 5.6 percent of the 30,455 people in solitary in prison.

Lawyers and inmates say some of the state’s 40,000 prisoners, including some with mental-health issues, have been kept in isolation for years, in one case for 14 years.

The Legal Aid Justice Center, which represents 12 inmates in isolation in Virginia, has requested an investigation by the U.S. Department of Justice, which recently launched a probe into a 1,550-bed Pennsylvania prison where inmates complain of long periods of isolation and a lack of mental health treatment.

Traylor said the majority remain in segregation for less than 90 days, and have no mental health issues,

But as more becomes known about the effects of isolation — on inmate health, public safety and prison budgets — some states have begun to reconsider the practice. Among them is Texas, which, like Virginia, is known as a law-and-order state.