"One of the medications that [Alexis] received does have a side effect that could in fact have been a problem," Miller told radio station WTOP Friday. "However, that doesn't mean that it was, and that's why we're asking the VA for a little more information as to what they prescribed, why they prescribed it, how much was prescribed."

According to The Washington Post, Alexis was prescribed Trazodone, which is frequently used to treat depression. However, the National Institutes of Health say it’s also meant to help with sleeplessness and schizophrenia. 

During the two visits Alexis made in August to emergency rooms at VA Medical Centers in Rhode Island and Washington, D.C., Alexis was only treated for insomnia.

On Wednesday, Miller requested that all of Alexis’s VA records be provided to the committee, in a letter to Secretary Eric ShinsekiEric Ken ShinsekiTrump VA pick faces challenge to convince senators he’s ready for job Is Ronny Jackson qualified to be the next VA secretary? Let's look at his predecessors GOP, vet groups react with caution to Trump VA pick MORE

The Navy Yard gunman registered with the VA in 2011 after he was discharged from the Navy. 

"What were the drug interactions? What were the side effects of the drugs that were prescribed by the VA?” Miller said to WTOP. “We do still have some questions that we're asking.” 

Miller spoke to VA officials this week, including Shinseki, and received a briefing with other members of the House and Senate Veterans’ Affairs Committees. 

The chairman was not satisfied with the VA’s response this week, according to a committee source. 

Miller also emphasized in his letter that none of Alexis’s records should be destroyed. 

"We want to make sure everything that could have been done was done, and that the VA does not do something to change the storyline."

Records indicate Alexis never sought an appointment from a mental health specialist, the VA has said. Alexis also either canceled or failed to show up at VA medical centers for primary care appointments and examinations required to submit disability claims.