A Dutch prosecutor has charged four suspects with causing the 2014 crash of Malaysia Airlines Flight 17 over Ukraine. The charges come five years after a missile took down the airliner and killed all 298 passengers and crew aboard, amid a wider conflict between the Ukrainian government and pro-Russian separatists in eastern Ukraine.

Speaking on behalf of an international investigative team, Dutch chief prosecutor Fred Westerbeke implicated three Russian nationals and one Ukrainian in the downing of the plane and the killing of those on board. “Even though they have not pushed the button themselves, there is suspicion that they have closely cooperated,” Westerbeke said. A video investigators played at the news conference said the men, who were all operating in eastern Ukraine at the time, “formed a chain” linking Ukrainian separatists to the Russian government.

Here’s what we know about the four suspects:

Igor Girkin, a.k.a. Igor Strelkov

Girkin, 48, went by the name Igor Strelkov when he commanded separatist forces in eastern Ukraine in spring 2014. A Russian national — and nationalist — Girkin previously served as a colonel in the Federal Security Service, or FSB, according to investigators. The FSB constitutes one of the Russian intelligence agencies that grew out of the Cold War-era KGB.

Girkin also formerly fought with Serbian forces in Bosnia and Russian forces in Chechnya.

Girkin commanded Russia-backed separatist forces during summer 2014 in the self-proclaimed Donetsk People’s Republic, where he functioned as the “so-called minister of defense,” according to investigators. In that role, Girkin implemented Stalin-era laws in the Ukrainian city of Slovyansk — and executed those who fell afoul of him.

The U.S. Treasury imposed sanctions on Girkin over his leadership in the separatist movement.

A Russian social media page linked to Girkin’s forces posted a message — which was later deleted — bragging that they had shot down a Ukrainian military aircraft around the time MH17 was shot down, fueling speculation that the separatists had hit the Malaysian Airlines passenger plane by mistake.

Families of 18 passengers aboard the MH17 sued Girkin for $900 million in a Chicago court, alleging that he masterminded the shooting down of the plane.

Girkin, in keeping with previous denials that eastern Ukrainian separatists were involved in the incident, told a Russian news agency Wednesday that his forces “didn’t shoot the Boeing down.”

Girkin is believed to be living in Moscow. Russia and Ukraine do not extradite their citizens to face charges abroad, so Girkin and the other three suspects will probably face trial in absentia.

Sergey Dubinskiy, a.k.a. “Khmuriy,” and Oleg Pulatov, a.k.a. “Guirza”

Dubinskiy, 56, works for the GRU, the Russian military intelligence service that has made headlines in recent years for allegedly interfering in the 2016 U.S. presidential election, poisoning double agent Sergei Skripal and sowing disinformation in Ukraine.

Dubinskiy was one of Girkin’s deputies and the head of the intelligence services in eastern Ukraine in July 2014, according to investigators. He apparently met Girkin while fighting in Chechnya in the mid-1990s.

According to a background video Dutch prosecutors aired Wednesday, Dubinskiy was in “regular contact with officials in Russia.”

Pulatov, 52, is a Russian national who served under Dubinskiy as deputy head of that intelligence service, according to the investigative team. He used to be a soldier with the Spetsnaz GRU, an elite military unit of the GRU.

Investigators said they think Dubinskiy and Pulatov also reside in Russia.

Leonid Kharchenko, a.k.a. “Krot”

Kharchenko, 47, is the only Ukrainian among the four suspects charged with downing MH17. He commanded a combat unit in the Donetsk region of eastern Ukraine in July 2014, according to investigators. Kharchenko reported directly to Dubinskiy.

Investigators said they assume he lives in a breakaway region in Ukraine not controlled by the Ukrainian government.