Man charged over US fertility clinic bombing

US authorities have arrested a man they say collaborated with the bomber of a fertility clinic, alleging he supplied chemicals used to make explosives and travelled to California to experiment with them in the bomber's garage.

The two men connected in fringe online forums over their shared beliefs against human procreation, authorities told reporters on Wednesday.

The blast gutted the fertility clinic in Palm Springs and shattered the windows of nearby buildings, with officials calling the attack terrorism and possibly the largest bomb scene ever in Southern California. The clinic was closed, and no embryos were damaged.

Guy Edward Bartkus of California, the bomber, died in the May 17 explosion. Authorities arrested Daniel Park, 32, of Washington state on Tuesday after he was extradited from Poland, where he fled to four days after the attack. Park is charged with providing and attempting to provide material support to terrorists.

Park spent years stocking up on ammonium nitrate, a chemical that can be used to make explosives, before shipping it to Bartkus and later visiting him in Twentynine Palms, California. He stayed for about two weeks earlier this year, and the two conducted bomb-making experiments in the detached garage of Bartkus' family home, said Akil Davis, the FBI's assistant director in charge.

Judge Cheryl Pollak ordered Park, 32, to remain detained, saying he posed a serious risk.

Authorities searched Park's home in Kent, a suburb of Seattle, and found large quantities of several chemicals and handwritten notes of chemical explosive equations, according to a federal complaint. One was for "an explosive recipe that was similar to the Oklahoma City bombing," Davis said, a reference to the 1995 explosion that killed 168 people and was the deadliest homegrown attack in US history.

Park shipped about 82 kilograms of ammonium nitrate to Bartkus in January and bought another 41 kilograms and had it shipped to him days before the explosion, authorities said.

Three days before Park visited him in January, Bartkus asked an AI chat application about explosives, detonation velocity, diesel and petrol mixtures, the complaint said. The discussion centered on how to create the most powerful blast.

Bartkus' relatives told investigators the two were "running experiments" in the garage. Bartkus' family did not raise any concerns to officials, authorities said.

Park and Bartkus, 25, 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 contends that people should not continue to procreate. Officials said Bartkus intentionally targeted the American Reproductive Centers, a clinic that provides services to help people get pregnant, including in vitro fertilisation and fertility evaluations.

Bartkus appeared to take responsibility for the attack on a website he set up that contained audio recordings, according to the complaint.

"Basically I'm anti-life. And IVF is like kind of the epitome of pro-life ideology," he allegedly said in one of the recordings.