SANTA ANA, Calif.—A Florida man pleaded guilty Jan. 19 for his part in the firebombing of a Planned Parenthood clinic in Costa Mesa nearly two years ago.

Xavier Batten, 21, of Brooksville, Florida, signed a plea agreement Tuesday, according to court records. He pleaded guilty in a federal courtroom in Santa Ana to possession of a destructive device and was scheduled to be sentenced May 15.

Co-defendant Chance Brannon, 24, of San Juan Capistrano, pleaded guilty Nov. 30 to conspiracy, malicious destruction of property by fire and explosive, possession of an unauthorized destructive device and intentional damage to a reproductive health services facility. The former Camp Pendleton-based Marine was scheduled to be sentenced April 15.

Co-defendant Tibet Ergul, 21, of Irvine, is awaiting trial.

Mr. Batten helped and conspired with Mr. Brannon to firebomb the clinic with a Molotov cocktail from February through March 13, 2022, according to his plea agreement.

Mr. Brannon and Mr. Ergul put together the Molotov cocktail about March 12, 2022, in Mr. Ergul’s garage, prosecutors alleged. Mr. Batten coached Mr. Brannon how to construct the fiery device, prosecutors said.

Mr. Brannon and Mr. Ergul ignited the device on March 13, 2022, and hurled it at the clinic’s entrance, sparking a fire, prosecutors alleged.

In May 2022, Mr. Batten discussed with Mr. Brannon about what it would take to bomb another clinic, prosecutors alleged in Mr. Brannon’s plea agreement.

Mr. Brannon advised Mr. Batten to put his “phone in airplane mode, plan ‘quick escape routes,’ avoid being identified on camera and obscure his face and skin color,” according to the plea agreement’s allegations.

After the Supreme Court’s ruling in June 2022 overturning the Roe v. Wade decision on abortion, Mr. Brannon and Mr. Ergul planned another firebombing of a clinic, but backed out at the last moment when they saw law enforcement, prosecutors said in the plea agreement.

Mr. Brannon and Mr. Ergul “also discussed starting a race war by attacking an electrical substation with the goal of damaging the substation and disrupting the functioning of the power grid in Orange County,” prosecutors said in the agreement.

Mr. Brannon also had a thumb drive that looked like a dog tag necklace bearing the U.S. Marine Corps motto, “Semper Fidelis,” which contained a file with a “plan and gear list for targeting a Southern California Edison substation,” prosecutors said.

One of the weapons in his possession was a Zastava ZPap M70 rifle with a racist message in handwritten Cyrillic on the folding stock, prosecutors said in the plea agreement.

It also “contained recordings of the 2019 Christchurch [New Zealand] mosque shooting, a mass shooting in which a white supremacist murdered 51 people and injured 40 others,” prosecutors said.

The duo were also accused of discussing and researching “how to attack the parking lot or electrical room of Dodger Stadium on a night celebrating LGBTQ pride,” prosecutors alleged.

Mr. Brannon and Mr. Ergul were both also charged with one count of possession of an unregistered destructive device and one misdemeanor count of intentional damage to a reproductive health services facility, a violation of the Freedom of Access to Clinic Entrances Act, prosecutors said.

The conspiracy and malicious destruction charges each carry a sentence of five to 20 years in federal prison, prosecutors said. The charge of possession of an unregistered destructive device is punishable by up to 10 years in federal prison. The intentional damage to a reproductive health facility charge carries a maximum sentence of one year in federal prison.

Authorities said the clinic “was forced to close and cancel dozens of appointments,” noting that clinic employees told investigators about 30 appointments had to be rescheduled.

Mr. Brannon agreed to pay about $1,000 in restitution for damage to the building.