(NewsNation) — A second American, Stephen Zabielski, has died fighting in the war in Ukraine, according to a State Department spokesperson.

According to Rolling Stone, the 52-year old stepped on a landmine. Zabielski was a U.S. Army veteran from upstate New York who had been living in Florida. His obituary said he died May 15 while fighting in a Ukrainian village. He leaves behind a wife, five stepchildren and seven siblings.

As word of his passing becomes public, the State Department says it is doing all it can to learn more about multiple Americans reported missing. For its part, Russia refuses to rule out the death penalty for those captured.

According to the Interfax news agency, multiple men are being held in a Russian-backed, separatist-controlled region of Donetsk where two British citizens and a Moroccan national were sentenced to death recently.

State Department officials said they are working to learn more about their capture and whereabouts.

Upon hearing the news, National Security Council spokesperson John Kirby warned Americans against fighting in Ukraine:

“We want to stress that this is not the place or the time for Americans to go to Ukraine. It is a war and if you want to help the people of Ukraine, there’s a whole lot of other, better options to do that than going and putting yourself in harm’s way in the middle of that war,” he said in a news conference Tuesday.

In late April, 22-year-old ex-Marine Willy Cancel Jr. became the first known U.S. citizen to die in the Russian conflict. And at least three Americans — all with military experience — are believed to be captured or missing.

Grady Kurpasi Jr. hasn’t been heard from since late April. The State Department does not have proof that he is alive.

But Alex Drueke and Andy Huynh, both Alabama residents, are believed to be in Russian custody, as they were seen in a video released by their captors.

Their families last heard from them June 8 and fellow fighters think they were taken prisoner the next day.

On Monday, Dmitry Peskov — a spokesman for Russian President Vladimir Putin — told NBC News he couldn’t guarantee that two U.S. military veterans captured in Ukraine would be safe from the death penalty.

Moscow’s stance is that foreign fighters in Ukraine are mercenaries, and thus not covered by the Geneva Conventions.

“It depends on the investigation,” Peskov said of Alexander Drueke and Andy Huynh, whose families reported them missing last week. Peskov went on to call them “soldiers of fortune who must be punished.”

“NewsNation Prime” had the mother of Drueke and Kurpasi’s father on the show Monday to respond to the news.

“A call to whoever is holding them: Treat them humanely. No matter who has him or Andy, because they may not believe in the Geneva Convention but they’re human beings and we expect them to treat our boys humanely,” Lois “Bunny” Drueke told “NewsNation Prime.”

“We believe my son is a prisoner of war,” added Grady Kurpasi Sr.

While yet to be accounted for, Grady Kurpasi Sr. believes his son, a U.S. Marine veteran, is the third American captured, as, he said, his son’s cellphone was tracked to a mall in Kherson that is reportedly being used as a Russian prison.

“That’s where we were able to locate him through nongovernment officials,” the elder Kurpasi said Monday. “If they have a chance to speak, that would be a wonderful thing. I would like to be able to see him televised or just have pictures and information about him.”

Lois Drueke told “Prime” that although Americans have been encouraged not to travel to Ukraine and engage in war, it was something her son felt called to do.

“Alex went, not as a representative of our country, he went because he truly feels that if Putin isn’t stopped, with every success he will become bolder and eventually begin threatening Americans,” she said. “And Alex might not be in the military anymore, but when he took that oath to defend and protect our country, he took it for life.”

Grady Kurpasi Sr. said his son was inspired to serve by the Sept. 11 attack.

“He found us and said he was going into the military and wanted to pay back to his country,” he said.

Ultimately, both parents just want their children home.

“Our world has been upside down, but there’s hope now that we realize he’s been captured — that he will be able to return to us,” Grady Kurpasi Sr. said.