LOS ANGELES (AP) — Two individuals who connected through niche online communities conducted explosive tests in the California desert which were later utilized to destroy a fertility clinic in Palm Springs, resulting in the death of one and the other fleeing to Europe. He was eventually detained and sent back to the U.S., federal officials revealed on Wednesday.

Daniel Park, aged 32, was apprehended on Tuesday night at New York’s John F. Kennedy Airport after being expelled from Poland. He had traveled there four days following the explosion, stated U.S. Attorney Bill Essayli during a press briefing.

Federal authorities allege Park collaborated with Edward Bartkus for months before Bartkus bombed the clinic last month and was killed in the blast.

Park was responsible for sending at least 270 pounds of a chemical substance used for making explosives to Bartkus. Earlier this year, he visited Bartkus’ residence in Twentynine Palms, California, where the two engaged in bomb-making experiments both in a room and a garage, explained Akil Davis, the FBI’s assistant director in charge.

Authorities searched Park’s home in Kent, a suburb of Seattle, and found “an explosive recipe that was similar to the Oklahoma City bombing,” Davis said.

Park shipped 180 pounds of ammonium nitrate to Bartkus and bought another 90 pounds of the chemical and had it shipped to Bartkus, authorities said. They did not elaborate on how he may have acquired the chemical.

Park and Bartkus met in online forums dedicated to the anti-natalist movement, bonding over a “shared belief that people shouldn’t exist,” Davis said.

Anti-natalism is a fringe theory that opposes childbirth and population growth and believes people should not continue to procreate. Officials said Bartkus intentionally targeted the fertility clinic as an act of terrorism. He tried to livestream the explosion, but the attempt failed, the FBI says.

The blast gutted the American Reproductive Centers fertility clinic in Palms Springs and shattered the windows of nearby buildings along a palm tree-lined street. Witnesses described a loud boom followed by a chaotic scene, with people screaming in terror and glass strewn along the sidewalk and street. Bartkus body was found near a charred vehicle outside the clinic.

Investigators haven’t said if he intended to kill himself in the attack or why he chose the specific facility. The clinic provides services to help people get pregnant, including in vitro fertilization and fertility evaluations.

Scott Sweetow, a retired ATF explosives expert, had previously said the amount of damage caused indicated that the suspect used a “high explosive” similar to dynamite and TNT rather than a “low explosive” like gun powder.

Those types of explosives are normally difficult for civilians to access, but increasingly people are finding ways to concoct explosives at home, he said.

“Once you know the chemistry involved, it’s pretty easy to get stuff,” Sweetow said. “The ingredients you could get at a grocery store.”

Davis previously called the explosion possibly the “largest bombing scene that we’ve had in Southern California.”