The US justice department has opened an investigation into the jail in Virginia where, the Guardian revealed, a young black man with mental health problems had died after spending months detained for stealing snacks worth $5.

Federal officials are investigating whether the civil rights of inmates are being violated at Hampton Roads regional jail in Portsmouth, where 24-year-old Jamycheal Mitchell was found dead in his cell last year.

“All prisoners, including those with mental illness, have a constitutional right to receive necessary medical care, treatment and services,” Vanita Gupta, the head of the department’s civil rights division, said in a statement.

Dana Boente, the US attorney for the eastern district of Virginia, said: “Prisoners with mental illness are a particularly vulnerable population, and their rights must be safeguarded.”

Mitchell died of heart problems and a wasting disorder in August last year, after a judge’s order that he should be committed to a mental health hospital was mislaid by a state official and forgotten about. Relatives said he suffered from schizophrenia and other psychotic problems.

He had been arrested by Portsmouth police more than three months earlier, for the alleged theft from a 7-Eleven store of a bottle of Mountain Dew, a Snickers bar and a Zebra Cake that were worth a total of $5. Mitchell’s family has sued authorities for $60m.

The jail has the highest death rate of any in Virginia. Subsequent deaths of two other inmates who had requested medical attention, Henry Stewart, 60, and William Thrower, 69, have come under scrutiny by the Virginian-Pilot and Richmond Times-Dispatch newspapers.

Amid mounting controversy about conditions at the jail, superintendent David Simons and assistant superintendent Eugene Taylor both stepped down from their jobs earlier this year.

A justice department spokesperson said officials would investigate whether inmates were being denied their constitutional rights to adequate medical and mental health care, and whether inmates with mental health problems were having their rights violated by being denied access to programs open to other inmates or secluded in isolation for prolonged periods.

They will look into the possibility that authorities at the jail oversaw a “pattern or practice” of violations of the Civil Rights of Institutionalized Persons Act and the Americans with Disabilities Act.

Gupta promised that the department would carry out “a thorough investigation, led by the facts and the law”.