FBI agents visited Larry Mitchell Hopkins's Vista Vista residence in New Mexico in November 2017 after hearing the alleged statement. Witnesses also reported seeing members of the militia with firearms in the house, it is said in the possible cause.
The FBI retrieved nine weapons and ammunition during the search.
Hopkins, 69, also known as Johnny Horton Jr., was charged on Monday with a crime involving firearms and ammunition stemming from the 2017 raid.
The leader of the United Constitutional Patriots was prohibited from possessing firearms or ammunition because he had previously been convicted of at least three crimes, including the criminal identity of a peace officer, according to the lawsuit.
The US Attorney's Office refused to say why Hopkins was tried more than a year after his residence was searched.
O'Connell said the weapons and ammunition did not belong to Hopkins.
"After the allegations I read, it's not what's happening down here," O Connell said, referring to the presence of the militia on the border.
The court document also details what witnesses alleged when contacting the FBI about Hopkins and the militia.
An FBI tip-line in October 201
Hopkins was identified as a so-called commander of the group, and his homeland was his "base," the court newspaper said.
Witnesses reported that about 20 militia members at Hopkins were "armed with AK-47 rifles and other firearms." Residence according to court documents.
"Hopkins allegedly also made the statement that the United Constitutional Patriots had been training to assassinate George Soros, Hillary Clinton and Barack Obama for supporting them by Antifa," court records said.
Soros is a billionaire investor and philanthropist.
The court papers did not say when Hopkins allegedly made the statement.
The armed group reportedly detained migrants near Sunland Park in New Mexico, federal prosecutors said.
Last week, videos were posted online allegedly held by militia migrants before being handed over to the US Border Patrol. In the films are people who wear full military fatigue, with pistols on the sides, wearing gloves and black masks.
Hopkins faces a maximum sentence of 10 years imprisonment if convicted and will remain in remand while awaiting a preliminary hearing on April 29
The Dakin Andone, Catherine Shoichet, and Chris Boyette of CNN contributed to this report.