By Dan Whitcomb and Steve Gorman

LOS ANGELES (Reuters) - A U.S. veteran of the war in Afghanistan who prosecutors say plotted to detonate a nail bomb at a Los Angeles-area white nationalist rally, hoping to cause mass casualties, was arrested in an FBI sting operation, federal prosecutors said on Monday.

Mark Domingo, 26, a U.S. Army infantryman who fought in Afghanistan and recently converted to Islam, was taken into custody on Friday after being given what he thought was a live bomb to use in the attack by an FBI informant, law enforcement officials said.

"Often we are asked what keeps us up at night. This is a case that keeps us up at night," Ryan Young, special agent in charge of the FBI's Joint Terrorism Task Force, told a news conference in Los Angeles.

Domingo, who had purchased several hundred long nails to serve as shrapnel in an improvised explosive device, had also suggested attacks on Jews, police officers, churches, a military facility, Southern California freeways and the Santa Monica Pier during internet conversations with the informant, Young said.

He was charged in a federal criminal complaint with providing material support to terrorists and was ordered held without bond during a brief initial appearance in U.S. District Court in Los Angeles on Monday.

“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 in a statement.

According to the criminal complaint, Domingo met the FBI informant in a private chat room in March, telling him he wanted to carry out an attack against America in retribution for the massacre of 50 people that month at two mosques in Christchurch, New Zealand.

'VIOLENT JIHAD'

During the initial conversations, he suggested that "America needs another Vegas event," an apparent reference to the 2017 mass shooting at a country music concert in Las Vegas where a lone gunman perched in a nearby hotel killed 58 people and wounded hundreds, the complaint alleges.

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Domingo, who said he wanted to become a martyr in a "violent jihad," also made reference to the 2013 Boston Marathon bombing, according to the complaint. During one meeting in person, he arrived carrying an AK 47-style assault rifle, telling the informant: "I just wanted to show you that I'm serious."

According to a law enforcement database, Domingo, who an FBI spokeswoman said received a less-than-honorable discharge from the Army, is the registered owner of three semi-automatic rifles.

Prosecutors say Domingo contemplated drive-by shootings and detonating a bomb at the Santa Monica pier before deciding to detonate an improvised explosive device at an April 28, 2019 rally in the Los Angeles suburb of Long Beach that he believed was organized by white nationalists.

Domingo and the FBI informant visited the planned site of the rally to scout the best location to place explosives to cause the greatest casualties, authorities said, agreeing that they would leave the scene separately to throw off suspicion.An FBI spokeswoman confirmed the rally took place as scheduled on Sunday.

The announcement of Domingo's arrest came two days after a gunman opened fire in a San Diego-area synagogue, killing one person and wounding three others. John Earnest, 19, was taken into custody in the shooting.

There was no indication that the two incidents were connected.

(Reporting by Dan Whitcomb and Steve Gorman; editing by Bill Berkrot, Tom Brown and Lisa Shumaker)