Federal prosecutors have begun presenting evidence to a grand jury in their criminal probe of at least 11 deaths suspected of being linked to improper insulin injections at the Veterans Affairs hospital in Clarksburg, W.Va., a sign that the 18-month investigation is intensifying, according to two people familiar with the matter.

The closed-door proceedings at the federal courthouse in Clarksburg began in early January with testimony from nurses and other medical professionals about the actions of the person of interest in the case, a former nursing assistant at the hospital who worked on the overnight shift, the people said. They spoke on the condition of anonymity because the investigation is ongoing.

Investigators think she gave the elderly veterans, most of whom were not diabetic, multiple, unauthorized injections of insulin that led to their deaths of severe hypoglycemia, or low blood sugar.

The deaths, which are being investigated as homicides, are another potential embarrassment for leaders of the sprawling and long-troubled VA system, which President Trump promised to reform. Reverberations from the case have reached the highest levels of the Justice Department.

Attorney General William P. Barr has at least twice since August called the U.S. attorney for the Northern District of West Virginia, Trump-appointee Bill Powell, to ask about the status of the investigation.

Such an inquiry from the top U.S. law enforcement official is unusual — particularly given the volume of politically sensitive matters on Barr’s plate, including the investigation of Trump lawyer Rudolph W. Giuliani, the review of how the FBI began its 2016 probe of the Trump campaign and the inquiry into the death in federal custody of multimillionaire sex offender Jeffrey Epstein. Barr, though, is known as an unusually hands-on attorney general.

He has also faced pressure from Veterans Affairs Secretary Robert Wilkie and Sen. Joe Manchin III (D-W.Va.), who have said publicly that hospital leaders in Clarksburg took too long to put the pieces together in the deaths, which initially were found to be of natural causes.

“A year and a half after these deaths, no one has been charged with a crime and we have yet to learn the details that led to these horrible deaths,” Manchin said in an email to The Washington Post. “Family members and Veterans across West Virginia deserve answers.”

A Justice Department spokeswoman declined to comment.

A person familiar with the matter said Barr became personally interested in the case in part because of Manchin’s involvement, and because of the possibly horrific circumstances of the case. Barr, the person said, is also often annoyed to see cases languish. The deaths have been under investigation since July 2018.

Veterans will be a key constituency for Trump as he runs for reelection with assurances that his administration fired poor performers at the agency and opened the door to more private care.

The Senate in December passed legislation sponsored by Manchin that would require all VA hospitals to report to Congress on improvements they’ve made to patient safety and quality of care. The Clarksburg hospital also would be required to report on what happened in the insulin case.

Although the convening of a grand jury is an indication that prosecutors are moving more aggressively in their investigation, it is not a definitive sign they will charge someone in the case. Federal grand juries are often used as investigative tools, allowing prosecutors to obtain subpoenas for documents or compel the testimony of witnesses.

The grand jury in West Virginia is expected to meet for weeks or months, according to people familiar with the matter.

“Attorney General Barr reached out to my office a few months ago for an update on this matter,” Powell said in a statement. “We have provided regular updates to the department on the progress of the investigation, which remains a high priority.”

It was not until Manchin’s public pressure that the U.S. attorney began taking aggressive steps to bring a case, according to a U.S. official familiar with the investigation. The prosecutor leading the probe recently left the office for a job in the private sector. Powell said in the statement that his office had “made this case a priority from the beginning and will continue to do so.”

In recent weeks, investigators have hired a team of outside medical experts to review the medical examiner’s findings, a common practice in cases with complex medical opinions.

The lengthy probe of what is largely a circumstantial case has become politically fraught. Wilkie has on multiple occasions blamed the drawn-out investigation on Veterans Affairs Inspector General Michael Missal, who was appointed by President Barack Obama and is jointly leading the probe with Powell.

“Well over a year after Clarksburg [hospital] reported this issue to authorities, Veterans and families are still waiting for the independent inspector general to complete its work and provide the closure West Virginia Veterans and families deserve,” agency spokesman Wesley Walls said in a statement.

The families blame hospital leaders.

“It’s been a year since they confirmed my dad was murdered, and VA still has not stepped forward to say what they have fixed to make sure this doesn’t happen again,” said Melanie Proctor. Her father’s body was one of seven disinterred last year and taken to Dover Air Force Base in Delaware for a forensic autopsy generally performed in cases of violent or suspicious deaths.

Felix McDermott’s blood sugar had plunged to dangerous levels hours before the retired Army sergeant died in April 2018 of hypoglycemia. “I’m tired of nothing happening with this case,” his daughter said.

At least 11 veterans died under similar circumstances on the ward known as 3A from the second half of 2017 through July 2018, authorities said.

“I don’t think we will ever know how many people were victims of these injections,” said Tony O’Dell, a Charleston lawyer representing several families in wrongful-death cases against VA.

Other criminal cases have engulfed the agency in the past year. About 140 miles south of Clarksburg in Beckley, W.Va., a former VA doctor is under investigation on allegations of sexually assaulting as many as 20 of his male patients, another case in which prosecutors have not brought charges. Last summer, a former VA pathologist in Fayetteville, Ark., was indicted on three charges of manslaughter after officials say he misdiagnosed thousands of patients while using drugs or alcohol. He has pleaded not guilty.

In Clarksburg, prosecutors have focused from the start on one person, according to people familiar with the case. She was assigned to monitor the veterans who died in what are known as one-on-one bedside vigils for patients who need extra attention. She was fired from the hospital last year, seven months after being removed from patient care.

The Post is not naming the woman because she has not been charged. She is now doing temporary work for a local construction company, a person familiar with her status said. Investigators have interviewed her multiple times, officials said.

Investigators have identified similarities in the deaths: Elderly patients in private rooms were injected in their abdomen and limbs with insulin the hospital had not ordered — some with multiple shots, according to people familiar with the case. The insulin, which was quickly absorbed, was given late at night when the hospital staff had emptied out. Within hours, the veterans’ blood-sugar levels plummeted.

Despite these common denominators, the medical staff and those with oversight of hospital procedures failed to identify a pattern for months.

After the bodies were exhumed, three deaths were ruled homicide by insulin injection, according to people familiar with the investigation. The armed forces deputy medical examiner changed the death certificates of the others from “natural causes” to “undetermined,” according to autopsy reports and people familiar with the case.

The deaths brought further scrutiny to VA’s internal controls. The hospital ward in Clarksburg did not have surveillance cameras, according to people familiar with the case, and the woman is believed to have had improper access to the medical supply room. The medicine carts on the floor also were routinely left unlocked.

Hospital officials have posted a board on 3A seeking suggestions from the staff for what areas of patient care need improvement.