Federal prosecutors have convened a grand jury to hear evidence in the case of a potential serial killer at a Veterans Affairs hospital in Clarksburg, West Virginia, according to an individual briefed on the case.

Investigators believe someone improperly injected 10 elderly veterans with fatal doses of insulin in 2017 and 2018. All were patients in the same ward and suffered severely low blood sugar before they died.

A hospital employee was fired, but no one has been charged. The individual briefed on the case spoke on condition of anonymity because of the ongoing and sensitive nature of the investigation.

The convening of a grand jury, first reported by The Washington Post, signals the investigation is ratcheting up after 18 months.

The case drew the interest of U.S. Attorney General William Barr in August, when it became public that two of the deaths had been ruled homicides.

“Attorney General Barr reached out to my office a few months ago for an update on this matter,” said Bill Powell, the chief federal prosecutor in northern West Virginia, in a written statement. He is leading the probe in conjunction with the VA's inspector general.

Barr “offered all the resources necessary for the investigation and I am grateful for his support," Powell said.

A person familiar with the matter who is not authorized to comment publicly said Barr was drawn by the scope of the investigation and its “horrific” nature.

Investigators have told family members they believe the assailant targeted elderly veterans.

Suspicious deaths:What we know about veterans who died at the Clarksburg VA

A lawyer for several families of the victims, Tony O’Dell, said they are “very pleased” that a grand jury has been convened. “It has been a long time coming,” he said.

His clients include the daughter of Felix Kirk McDermott, 82, one of three veterans who died in April 2018 after suffering unexplained severe low blood sugar. McDermott and a veteran who died the next day, George Nelson Shaw, 81, were not diabetic.

Insulin is used by diabetics to keep blood sugar levels in check, but it can be lethal for nondiabetics.

Former U.S. attorney: 'This could take months'

William “Bill” Ihlenfeld, who preceded Powell as U.S. attorney, said a special grand jury probably had been convened, given the number of victims and volume of evidence to be gathered.

Federal grand juries in the district typically meet one day every two months for a year to consider cases of virtually every stripe, said Ihlenfeld, now a West Virginia state senator. Special panels often meet more frequently, even several times a week, to focus exclusively on one investigation.

“This is not going to be a grand jury that will meet on Tuesday, followed by the announcement of an indictment on Thursday,” Ihlenfeld said. “This could take months.

“More than likely, you’ll have staff from the hospital, FBI agents and voluminous medical records to review for each and every victim. This could be classified as a death penalty case, so they (prosecutors) will need to get it right. This will take time.”

Ihlenfeld said Barr’s interest in the case is unusual. In six years as U.S. attorney in the Obama administration, he said, he was never asked to brief the attorney general on a case.

But he acknowledged the case is unusual, too, and it's in one the smallest federal districts in the country.

“This is happening to veterans, who have always represented a special class of people, and it has brought political pressure from Sen. (Joe) Manchin,” Ihlenfeld said.

Manchin, D-W.Va., introduced legislation in December that requires the VA to report what steps it has taken to improve patient safety at its hospitals and prevent similar deaths in the future. The bill passed the Senate and is pending in the House.

“I’ve been calling on DOJ and VA OIG to complete this investigation because the victim’s family members and veterans across West Virginia deserve answers,” Manchin said in a statement. “I’m encouraged to see some forward momentum.”

'National tragedy':Surveillance, staffing under review after deaths at VA hospital

Lawyer for families says VA hasn't offered much information

O'Dell, the lawyer for several of the families, said that while he welcomes the developments in the case, his clients want more information from the VA about what happened.

“Unfortunately, the VA has not provided any answers or evidence of accountability for those within the VA that allowed this nightmare to go on for as long as it did,” O’Dell said. “It is long overdue.”

USA TODAY reported in October that hospital staff missed opportunities to figure out what was happening, which may have risked veterans' lives and limited evidence in the probe.The hospital didn't adequately track insulin, and there were no surveillance cameras on the ward, according to employees.

By the time hospital officials notified authorities in June 2018, at least eight patients had died under suspicious circumstances. Several had been embalmed and buried, destroying potential evidence. One veteran had been cremated.

Months after McDermott and Shaw died, investigators had their bodies exhumed. A military medical examiner detected injection sites and evidence of insulin. He ruled both deaths homicides.

Wesley Walls, spokesman for the Clarksburg VA hospital, said in a statement Thursday that hospital staff detected the suspicious deaths. "Clarksburg also fired the individual at the center of them," he said. The hospital notified VA headquarters, which tipped off the VA inspector general.

Officials at the VA, from Walls up to the secretary of the agency, have criticized the inspector general's office for taking so long to investigate the deaths.

Asked to comment Thursday, a spokesman for the VA Office of Inspector General pointed to a statement issued in August saying the office was investigating the deaths but couldn't comment further.

Powell, the U.S. attorney, said the investigation is "ongoing and active.”

“We have made this case a priority from the beginning," he said, "and will continue to do so."