(CNN) The American paramedic killed during a monitoring mission in Ukraine has been identified.

He is Joseph Stone, 36, according to the Organization for Security and Cooperation in Europe's Special Monitoring Mission.

It is the first time a member of an SMM patrol has been killed while on duty.

"I was absolutely shocked. I always knew it was a possibility," Matthew Stone told CNN, reacting to his brother's death. "He always knew something like this could happen."

Stone died and two other monitors were injured on Sunday after an explosion damaged a mission vehicle near Pryshyb in eastern Ukraine. A mine might have triggered the blast.

The injured monitors, a German woman and a Czech man, were transported to Kramatorsk on Monday and will be taken to Kiev where they will receive "appropriate care," said the mission's spokeswoman Alexandra Taylor.

Fighting continues between pro-Russian separatists and Ukrainian armed forces in eastern Ukraine, according to OSCE Special Monitoring Mission Chief Monitor Ertugrul Apakan said Monday.

"I reiterate my call for sustainable cease-fire, withdrawal of weapons, full demining and real commitment to peace. And I ask that those responsible for placing mines are held accountable," Apakan said.

2 SMM monitors, female from Germany & male from Czech Republic, are stable, released from #Luhansk hospital, will be moved to Kyiv — OSCE SMM Ukraine (@OSCE_SMM) April 24, 2017

'He loved it'

Joseph was born in Milwaukee in March 1981, but lived in Arizona since an early age. He liked to be known as Joe but "growing up we always called him Bobby," Matthew said.

His middle name was Robert, the name of an uncle -- Bob -- who was a firefighter and a paramedic.

He went to Pima Community College in Tuscon, where he got his certifications as an EMT and then as a paramedic. He plied his trade at Southwest Ambulance service.

For the last five years or so he worked overseas, as a contractor for various companies.

Joseph Stone was 36.

He worked in Afghanistan for a while and went to Liberia for about a year, his brother said. He also did contract work on seismic vessel in the North Sea. After that, he spent time in Iraq.

Matthew said his brother got his final job with the Global Rescue company and worked in Ukraine with the OSCE monitoring mission.

He'd work two months or so in Luhansk and then come back for a month. He rotated back and forth several times, Matthew said.

As for Ukraine, he said he's met a lot of nice people there and felt bad for them.

"You see all these bombed out buildings."

Toward the end, Matthew said, he wasn't liking the travel aspect of the job.

But, "he liked the action. He loved it. He loved it," Matthew said.

Wanting to make a difference

The Special Monitoring Mission is an unarmed civilian mission tasked with working to help normalize and stabilize the situation in eastern Ukraine.

They'd drive around and look to see who is violating the Minsk Agreement and got their share of harassment.

"That was their mission to drive around and report the facts," Matthew Stone said. "If somebody got injured, it was his job to treat them."

"He liked going to those places where he could make a difference," Matthew Stone said. "He liked going where people needed him. He couldn't not try to help somebody. That's just who he was."

Matthew describes his brother as a "secular activist" and a "humanist" and had a great passion for thinkers in the secularist and atheist traditions.

He was a "voracious reader" of Christopher Hitchens, Richard Dawkins, Sam Harris, Lawrence Krauss, and Maajid Nahwaz.

"The world," Matthew said of his brother, "is a lot worse off without him."