On May 2, 2014, Ukrainian nationalists locked pro-federalism protesters in Odessa's Trade Unions House and set the building on fire. Almost 50 people died and 250 others were injured in the attack by the Ukrainian radicals, according to the United Nations.
"It was well orchestrated," Gagnon said on Wednesday.
The CIA as well as other US agencies were likely involved in the planning and execution of the massacre, he said.
Gagnon pointed out that the perpetrators were well-equipped to carry out the massacre and Ukrainian law enforcement did practically nothing to prevent it.
"Molotov cocktails were on hand to firebomb the Trade Union House. Bats were on hand to beat those who jumped from windows trying to get away from fire and smoke. Back door was broken into, and Nazis moved inside to kill people hiding," Gagnon said.
The police at the scene did nothing while the assault was underway and arrested those who had been inside the building but not those who attacked them, Gagnon said.
Fire trucks were blocked from arriving at the scene for quite some time and could not extinguish the fire in the very beginning, Gagnon added.
"It is obvious to me that decisions made at the highest level in Ukraine, and likely in the United States, determined that no prosecutions would happen to those who attacked," Gagnon said.
The activist noted that he visited Odessa on May 2, 2016, and saw with his own eyes the neo-Nazi Azov* battalion holding weapons and roping off the Trade Unions House from people who carried flowers to pay tribute to the victims of the attack.
"No one was allowed to approach the building. The Nazis threw stones at the bus carrying the mothers [of the victims] and the international guests as we approached the area," Gagnon added.
The clashes in Odessa became one of the deadliest events during the so-called Maidan and anti-Maidan demonstrations in Ukraine that started in late 2013.
Moscow has repeatedly criticized the steps Kiev took to allegedly investigate the massacre and has urged the international community and human rights groups to conduct a probe.
*Banned as a terrorist organization in Russia