The 22-year-old was working with a private military contracting company when he was killed on Monday. The company had sent him to Ukraine, and he was being paid while he was fighting there, Cancel's mother, Rebecca Cabrera, told CNN.

Cancel, a former US Marine, according to his mother, signed up to work for the private military contracting company on top of his full-time job as a corrections officer in Tennessee shortly before the war in Ukraine broke out at the end of February, Cabrera said. When the war began, the company, according to Cabrera, was searching for contractors to fight in Ukraine and Cancel agreed to go, Cabrera said.

"He wanted to go over because he believed in what Ukraine was fighting for, and he wanted to be a part of it to contain it there so it didn't come here, and that maybe our American soldiers wouldn't have to be involved in it," Cabrera told CNN in a phone interview.

Cancel flew to Poland on March 12 and crossed into Ukraine sometime over March 12 and 13, Cabrera said.

The group of men Cancel was fighting alongside were from "all different countries," Cabrera said.

Cabrera was told by those who notified her of her son's death that his body had not been found.

"They haven't found his body," she said. "They are trying, the men that were with him, but it was either grab his body or get killed, but we would love for him to come back to us."

A State Department official said they are "aware of these reports and are closely monitoring the situation."

"Due to privacy considerations, we have no further comment," the official added. "We once again reiterate US citizens should not travel to Ukraine due to the active armed conflict and the singling out of US citizens in Ukraine by Russian government security officials, and that US citizens in Ukraine should depart immediately if it is safe to do so, using any commercial or other privately available ground transportation options."

Cancel leaves behind a 7-month-old baby and a wife, Cabrera said. Cancel was living in Tennessee before he left for Ukraine, and is originally from Orange County, New York, said Devin Tietze Jr., Cancel's brother-in-law.

Cabrera said Cancel "always put everybody ahead of himself."

"He was just a really thoughtful person. He always put everybody ahead of himself even when situations were so stressful," Cabrera said of her son. "He always kept everybody laughing and calm. He was the man that stood up when everybody else stood back."

Tietze said Cancel was the "type to fight for what's right regardless of the outcome," when asked why Cancel had decided to join the fight in Ukraine.

"He believed wholeheartedly this shouldn't have happened and he wanted to go help the people in Ukraine," Tietze said in a phone interview.

Cabrera said Cancel "wanted" to do the military contract work.

"It was something that he believed in his heart, that was the right thing. He was the type of man who always stepped up when everybody else stepped back, and there were a lot of men who were like that that were with him," Cabrera said.

Earlier on Thursday the British government confirmed a UK citizen had been killed in Ukraine and said officials are "urgently seeking further information" on another who is missing, a UK Foreign Office spokesperson told CNN.

"We can confirm that a British national has been killed in Ukraine and are supporting their family," they said.

The Ukrainian Defense Ministry created a special unit, the International Legion, for foreign fighters who wanted to join the fight against Russia. More than 20,000 volunteers and veterans from 52 countries had expressed their desire to join the fight as of March 7, according to Brig. Gen. Kyrylo Budanov, commander of the Main Intelligence Directorate of the Ministry, which will run this legion.

CNN previously reported on foreign nationals, including citizens from the US and the UK, who had left home to join the fight in Ukraine.