A U.S. Army veteran in Reseda has been charged with plotting a terror attack on a white supremacist rally in Long Beach as revenge for the mosque shootings in New Zealand after FBI officials thwarted his plans through an undercover sting, authorities said Monday.

Mark Steven Domingo, a 26-year-old former soldier who served four months in Afghanistan and recently converted to Islam, was arrested Friday after receiving what he believed was a live bomb from an undercover officer posing as his co-conspirator, the U.S. Department of Justice said in a news release.

He met the officer last month in a private online messaging group where he called for violence on American soil, later discussing targeting Jews walking to synagogue, law enforcement officials and a military facility with the officer, prosecutors said.

According to federal authorities, the pair met multiple times in recent weeks and went over details of the planned attack, from the Long Beach park where it would take place to the nails that would be used for shrapnel.

“Domingo said he specifically bought three-inch nails because they would be long enough to penetrate the human body and puncture internal organs,” the affidavit states.

The rally in Long Beach that Domingo was allegedly targeting was expected to be a gathering of white supremacists but police said they ultimately decided not to show up. Video shows a group of counterprotesters arrived to the location instead, with some seen holding signs such as “Be Best, Not Racist.”

In effort to keep open communication w/our community, we want to inform you #LBPD received a call from an individual claiming to be the leader of UPNF stating the group will not be at today’s demonstration. LBPD will still deploy in the area to ensure the safety of our community. https://t.co/b6oHZ6yGOV

— Long Beach PD (CA) (@LBPD) April 28, 2019

Authorities said there was a serious chance Domingo could have carried out what prosecutors have described as a “chilling” terror plot.

“That’s the risk,” said Ryan Young, the FBI special agent in charge of the case. “In some of these violent chatrooms, they find a lot of like-minded individuals.”

“This is a case that keeps us up at night,” Young said.

Domingo appeared in court Monday afternoon and his family issued a statement to KTLA asking for privacy and saying they have ill relatives requiring their full attention.

“We do not know what is going on at this point. We are surprised by all of these events in regard with Mark,” the statement reads. “I hope you will respect our privacy in this matter, as we are distressed by all of this and don’t need all of this added attention.”

The suspect’s younger brother also spoke to reporters outside the family home in Reseda, explaining Domingo had converted to Sunni Islam sometime late last year or as recently as earlier this year.

“Like anybody else, I don’t want to assume a thing when someone joins a new religion. If anything, I thought it was a good thing. I thought maybe my brother finally found some sort of guidance in this world,” James Domingo said of his older brother.

But, according to federal prosecutors, the older Domingo descended into what they have described as a “rapid radicalization” in support of jihad rather than just religious conversion.

James Domingo declined to say whether he believed his brother was guilty, saying he would “wait for the trial.”

According to Young, Domingo was also planning a “backup” attack at another location only described as a “similar rally” in Huntington Beach. Meanwhile, U.S. District Attorney Nick Hanna said Domingo also mentioned trying to target crowds at the Santa Monica Pier or killing one of his neighbors as “a prelude to a much larger attack.”

But FBI officials said the public was not in danger and there was no credible threat in the two months Domingo was allegedly plotting.

Last week, Domingo purchased several hundred nails to be used as shrapnel for a bomb, giving them to the undercover officer and sending another message Thursday saying the terror plot would proceed, according to the affidavit.

On Friday, he received multiple inert devices he believed to be weapons of mass destruction from the undercover officer, prosecutors allege. He inspected the devices and then went to the park where the attack was planned to surveil the location.

Authorities took him into custody shortly after.

As a U.S. infantryman with combat experience, Domingo was initially assigned to Fort Campbell in Kentucky before being deployed to Afghanistan from September 2012 to January 2013, federal authorities said.

He first came to the attention of authorities last month when FBI officials spotted his writings in a private message group calling for an attack on American soil and supporting violent jihad.

On March 3, 2019, he wrote: “america needs another vegas event tbh something to kick off civil unrest….,” according to federal authorities.

His writings allegedly continued: “its not about winning the civil war its about weakening America giving them a taste of the terror they gladly spread all over the world.”

After the March 15 mosque attacks in New Zealand left 50 people dead, Domingo wrote: “there were mosque shootings in new Zealand [sad emoji] … there must be retribution,” according to prosecutors.

Upon seeing that last post, an undercover FBI official started a private online conversation with Domingo, authorities said, in which they discussed their anger about the mosque attacks.

Another undercover operative in the same messaging group then also contacted Domingo on March 16.

According to federal prosecutors, Domingo went on to say he’s been keeping an AK-47-style rifle near his bed in the wake of the mosque shootings. Authorities said he has three firearms legally registered in his name, including that gun and two other rifles.

Domingo later suggested he and the undercover operative meet in person and they attended a prayer service on March 18 — an interaction that was audio recorded, according to federal authorities. It was then that Domingo allegedly discussed attacking Jews, police officers, a military facility and churches, the affidavit alleges.

But federal prosecutors and FBI officials said the terror plot planned for Sunday actually came into shape later during an April 19 meeting. Domingo allegedly showed up to meet the undercover operative while armed with an AK-47-style rifle “to show you that I’m serious,” according to the criminal complaint.

He allegedly referred to the Boston massacre during the meeting and asked for someone who could construct an IED that would cause 50 casualties.

Authorities are continuing an investigation into Domingo, including a probe of his social media accounts, and have called for the public’s help in providing any possible tips.

“This investigation successfully disrupted a very real threat posed by a trained combat soldier who repeatedly stated he wanted to cause the maximum number of casualties,” Hanna said in a statement from the DOJ.