Domingo allegedly bought parts, including nails, for an improvised explosive device that would be remotely triggered, but in fact contained inert materials, Federal Bureau of Investigation agent Tasha Coolidge said in an affidavit filed in US District Court in Los Angeles.
A message left on a phone listed for Domingo was not immediately returned.
Authorities added that after Domingo considered attacks targeting "Jews, churches and police officers", he made a decision to detonate an improvised explosive device at the Long Beach rally, which police said was called off Sunday morning.
Federal and local law enforcement agencies said they thwarted a terror plot targeting white supremacists in California by an Army veteran who converted to Islam and wanted retribution for the New Zealand mosque attacks. He asked the bomb-maker to create an IED that could cause up to 50 casualties and referenced the Boston Marathon bombing.
Domingo, an infantryman who served a combat stint in Afghanistan in 2013, has been charged with providing material support to terrorists. He was referring to the October 2017 mass shooting in Las Vegas.
"All of these plans were related to Mr. Domingo's stated belief in violent jihad and Americans should pay for attacks on Muslims around the world", Hanna said, noting Domingo wrote "There must be retribution", following the mass shootings at a mosque in New Zealand.
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Domingo also, according to the affidavit expressed support for the Islamic State leader Abu Bakr al-Baghdadi.
The Justice Department release said that after that post, the undercover operative began to engage Domingo in online conversations and later met him in person.
The planned rally at Bluff Park did not occur, but there were counter-protests that day.
"I'm extremely glad to be announcing that we interdicted a potential terrorist attack, rather than outlining the FBI's response to yet another tragedy", said Paul Delacourt, assistant director in charge of the FBI's Los Angeles Field Office.
"It's unclear how far the plot got, but sources said the man had specific targets, including Los Angeles police officers, the Santa Monica pier and a location in Long Beach".
News of the foiled terror plot comes just two days after a gunman entered a synagogue in San Diego County and opened fire, killing one woman and wounding three others in what officials called an anti-Semitic hate crime.