Charleston, West Virginia — The Department of Justice confirmed this week it has joined the investigation into suspicious deaths at a West Virginia Veterans Affairs medical center. At least 10 veterans may have died from wrongly administered insulin injections.

Sisters Melanie Proctor and Chris Niehenke are seeking justice for the 2018 death of their father, retired Army Sergeant Felix Kirk McDermott. Niehenke said their father was 82 when he died, and they "just thought it was his time."

That was until the family said an FBI agent showed up at their home, requesting permission for McDermott's body to be exhumed for further investigation. The sisters said his autopsy revealed homicide by insulin injection, something that can be deadly for non-diabetics.

"I can't imagine what he felt like, it's just beyond my comprehension," Niehenke said.

Felix McDermott Family photo

In addition to McDermott, Air Force veteran George Shaw, who died just a day later, is the second confirmed homicide, according to an attorney for the Shaw family.

"If they had multiple deaths, why didn't they catch it sooner? It's not like dad was one of the last people to pass because of this," Niehenke said.

"Somebody dropped the ball somewhere on catching this, before it got as far as it did," Proctor said.

CBS News has learned investigators have approached a third family requesting permission to exhume another veteran's body.

West Virginia Senator Joe Manchin said his office first learned last summer an inspector general's investigation had been launched. But Manchin said he did not know foul play was suspected.

"For the Veteran's Administration to hold this back, or for the local administration to the VA not let those of us who represent the people concerned, you can imagine the panic that's going on right now. I know of operations that people refuse to have that have been scheduled for a long time. They're concerned about the care," Manchin said, also telling CBS News that multiple families have reached out to his office asking for help in getting answers.

While a VA spokesman said the allegations do not involve any current employees, a person of interest has been identified. Proctor and Niehenke said that brings little comfort until charges are filed and others are held accountable.

"It is a betrayal because these guys trust that place, we trusted them, we trusted them to take care of dad," Proctor said.

She said it will be hard to accept what happened to her father.

"You want to know what happened, you want to know who to hold responsible for it," Proctor said. "You lose all trust in the system when this happens."

The family of McDermott are the first to file a wrongful death claim, which really brought this issue into the public spotlight more than a year after the government investigation into this began. An Attorney for the Shaw family, the second confirmed homicide, said they're also considering a claim.