An Army veteran who converted to Islam and discussed launching various terror attacks throughout Southern California was arrested as he plotted to bomb a white supremacist rally as retribution for the New Zealand mosque attacks, federal prosecutors said Monday.

Mark Domingo, an infantryman who served a combat stint in Afghanistan, was arrested Friday after visiting a park in Long Beach where authorities said he planned to plant home-made explosive devices made with nail-filled pressure cookers in advance of a Nazi rally scheduled Sunday.

Domingo, 26, was arrested on a charge of providing material support to terrorists. A criminal complaint said he had been planning since March to "manufacture and use a weapon of mass destruction in order to commit mass murder."

U.S. Attorney Nick Hanna told a news conference that Domingo supported violent jihad and spoke about becoming a martyr and of pledging allegiance to the Islamic State group if it established a presence in the United States.

"This is a case in which law enforcement was able to identify a man consumed with hate and bent on mass murder, and stop him before he could carry out his attack," Hanna said. "The criminal case outlines a chilling terrorism plot that developed over the past two months and targeted innocent Americans that he expected to gather this past weekend."

Investigators said Domingo posted an online message March 3 that said "America needs another Vegas event," an apparent reference to the 2017 mass shooting that killed 59. He allegedly said it would spark civil unrest to weaken "America by giving them a taste of the terror they gladly spread all over the world."

After Domingo began to discuss seeking revenge for the March 15 attacks that killed 50 people at New Zealand mosques, a confidential source who has worked with the FBI since 2013 began chatting with him about his plans.

Domingo said he had an assault rifle, semi-automatic rifle and several magazines of ammunition. He discussed picking off Jews walking to synagogue, shooting police officers or attacking a church or military base, according to investigators.

When the source asked him how he planned to carry out violence without getting caught, court records said Domingo replied: "Martyrdom, bro."

He also allegedly discussed killing a neighbor he was upset with as a prelude to broader violence and later contemplated bombing the Santa Monica Pier, where he said a summer attack on the crowded tourist spot would maximize casualties because people wouldn't be able to escape the blast in the enclosed space.

Eventually, he settled on the idea of planting an improvised explosive device that would be remotely triggered or detonated by a timer at a white supremacist event, investigators said. The confidential source connected him with a purported bomb maker who was actually an undercover police officer.

Domingo bought 8 pounds (3.6 kilograms) of nails long enough to puncture internal organs and provided them to the bomb maker, though the resulting contraption contained only inert materials, authorities said.

If he survived the planned bombing, Domingo allegedly discussed launching further attacks on the Long Beach Port or on a train.

The plot was thwarted just two days ahead of the event, investigators said, when an FBI SWAT team arrested Domingo after he was given the pressure cookers and surveyed Bluff Park in Long Beach, where the rally was supposed to happen.

White nationalists, however, never showed up at the park Sunday. Instead, a large group of counter protesters demonstrated for peace.

Phone and email messages seeking comment from the public defender representing Domingo were not immediately returned. A voicemail left at a phone number listed for Domingo was not returned.

A statement from his family asked for privacy.

"We do not know what is going on at this point," the statement said. "We are surprised by all of these events in regard with Mark."

The suspect's younger brother told reporters outside the family's Los Angeles home that Mark Domingo had converted to Islam in recent months.

"Like anybody else, I don't want to assume a thing when someone joins a new religion," James Domingo said. "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 declined to speculate on whether his brother was guilty, only saying he would "wait for the trial."

Domingo is believed to have acted alone and authorities said the public was not at risk. After Domingo was identified online by the FBI, he was under surveillance around the clock, said Ryan Young, the FBI special agent in charge of counter terrorism at the Los Angeles office.

"Our biggest fear is this was a rapid what we call 'radicalization to mobilization to violence,'" Young said. "Sometimes we get asked 'What keeps you up at night?' This is a case that keeps us up at night."

While Domingo had appeared committed to carry out the plot, court papers portray him as suddenly reluctant as the bomb plot came together.

He told the others he thought were his confederates they should consider postponing so he could finish reading the Quran and experience the Muslim holy month of Ramadan beginning in May.

Last Wednesday, he told the confidential source he wanted to think about it for a night.

"Let's just sleep on it," he said, according to court papers. "If we're still as motivated ... I'll give the go-ahead."

The next day he sent a message to the supposed bomb maker that the plan was on. The two referred to the bombs as "presents" they needed to wrap for a party.

"Keep the presents somewhere safe lol," Domingo wrote back Friday morning before making plans to meet that night. (AP)

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