LOS ANGELES (AP) Two men who met in fringe online forums tested explosives in the California desert that were later used to blow up a fertility clinic in Palm Springs, killing one of them and sending the other fleeing to Europe, where he was arrested and extradited to the U.S., federal authorities said Wednesday.

Daniel Park, 32, was taken into custody on Tuesday night at New York’s John F. Kennedy Airport after being deported from Poland, where he traveled four days after the bombing, U.S. Attorney Bill Essayli told reporters.

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

Park shipped at least 270 pounds of a chemical compound used as an explosive precursor to Bartkus and traveled earlier this year to Bartkus’ home in Twentynine Palms, California, where the two conducted experiments in bomb-making in his room and a garage, FBI’s assistant director in charge Akil Davis said.

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.”