The brother of a US Army veteran suspected of plotting terror attacks in California said on Monday he thought his sibling's conversion to Islam was a 'good thing' and would have helped him find 'guidance.'

Mark Domingo, 26, an infantryman who served in Afghanistan, was arrested on Friday after visiting a park in Long Beach, Los Angeles, where authorities said he planned to plant home-made explosive devices made with nail-filled pressure cookers in advance of a Nazi rally scheduled on Sunday.

Federal prosecutors said Domingo converted to Islam and wanted retribution for the New Zealand mosque attacks.

The suspect's younger brother, James Domingo, spoke out about his brother's arrest outside the family's Los Angeles home.

'I never thought my brother would be charged with any of this,' James told reporters. 'This whole thing is surprising to us to all the Domingo family and to our relatives,' the 22-year-old added.

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James Domingo, 22, the brother of U.S. Army veteran, Mark Domingo, 26, who is suspected of plotting terror attacks in California said he's surprised by his sibling's arrest. 'I never thought my brother would be charged with any of this,' James (pictured) said

Prosecutors said Monday that Mark Domingo (pictured), 26, was arrested on Friday for planning to plant a bomb at a Nazi rally Sunday in Long Beach. He allegedly told an FBI informant that he wanted to kill officers and set off explosives along freeways in Los Angeles

Mark Domingo was an infantryman who served a combat stint in Afghanistan. He is pictured left in 2012 taking an Afghan man's fingerprints as part of the military's effort to gather biometric data on the residents

James said he thought that his brother's religious conversion to Suni Islam would help him.

'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 found some sort of guidance in this world,' James said

James declined to say whether he believed his brother was guilty or innocent. He said he would 'wait for the trial'.

His family released a statement on Monday, saying they had no further comment about the criminal charges against Domingo and asking for privacy.

Mark Domingo, who was arrested on a charge of providing material support to terrorists, had allegedly been planning to 'manufacture and use a weapon of mass destruction in order to commit mass murder' since March.

Domingo allegedly 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.

Domingo (pictured) has been charged with providing material support to terrorists

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 informant who has worked with the FBI since 2013 began chatting with him about his plans.

'There must be retribution,' he allegedly wrote in one online post about the New Zealand mosque attack.

'I feel like I should make a Christian's life miserable tomorrow for our fallen bros (and) sis in New Zealand... maybe a Jew's life... they shed our blood... no Muslim should have to experience this, a message needs to be sent,' he wrote in another post.

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.

He allegedly made references at different points to the 2013 Boston Marathon bombing and 2017 shooting rampage at a Las Vegas country music concert.

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

At another point, Domingo reaffirmed that he was 'not afraid to die a martyr'.

He also allegedly discussed killing a neighbor he was upset with as a prelude to broader violence.

Domingo allegedly had plans to plant the bomb at a Nazi rally on Sunday in Long Beach

White nationalists never showed up to the planned event in Bluff Park, but a large group of counter protesters (pictured) demonstrated

He allegedly told the informant that he thought about blowing up popular attraction, Santa Monica Pier (pictured above)

He later also allegedly contemplated bombing the Santa Monica Pier because 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 informant connected him with a purported bomb maker who was actually an undercover police officer.

Domingo bought 8 pounds of three inch nails that were long enough to puncture internal organs and provided them to the bomb maker. 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.

As part of the plot, Domingo asked the informant to find a bomb-maker and get them to write down the instructions and 'ingredients'.

'I'll be honest. I'm smart in history but I failed chemistry. Science is not my forte... If he doesn't want to make it, don't force him,' he allegedly told the informant. 'But ask him how. Details... F**king to the letter details. Ask him.'

Domingo noted that they would have to buy the bomb elements little by little from various hardware stores.

'You can't buy these ingredients en masse, you know that... because that's more suspicious than buying guns,' he said.

'The average person can make explosives... It's not easy but it's not hard. If they put the time into it, everyone can make explosives. But obviously... they don't want to because, you know, why would you? Only a terrorist would want to, right?'

FBI Special Agent in Charge Ryan Young briefed the media on Monday beside photos of a suspect arrested in connection with an alleged terror plot targeting Southern California sites

Domingo, who allegedly arrived at one meeting with the informant armed with an AK-47-style assault weapon, said he wanted to specifically make an improvised explosive device to detonate at the crowded rally.

'Even a small IED would do damage in a crowd... You're looking at at least 20 people dead, maybe even 30 people injured. I think if we go the IED route we are going to storm,' he said.

After Domingo provided the nails to the undercover operative for use in the construction of the bomb, Domingo sent a message on Thursday indicating that the operation was to proceed, according to the affidavit.

On Friday night, the undercover operative delivered multiple inert devices, which Domingo believed were weapons of mass destruction.

Domingo was arrested after inspecting the devices and visiting the planned site of the rally to scout the best location to place explosives to cause the greatest casualties.

Domingo could face a maximum sentence of 15 years in federal prison if convicted.

'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,' U.S. Attorney Nick Hanna said.

'Protecting Americans from terror attacks is the number one priority of the Justice Department, and anyone who plots to use a weapon of mass destruction will be held to account.'

The news of Domingo's alleged domestic terror plot comes less than 48 hours after John Earnest, 19, was arrested for allegedly shooting dead one woman and injuring three others at a synagogue in San Diego.

Earnest was identified by police as the gunman who burst into the Chabad of Poway near San Diego on Saturday, the last day of Passover, and opened fire with an assault-style rifle.

About 100 congregants were worshiping when the gunman killed Lori Kaye, 60, and wounded the synagogue's rabbi, Yishoel Goldstein; eight-year-old Noya Dahan; and her uncle Almog Peretz.