A self-proclaimed white supremacist has been arrested after allegedly making online threats toward the jury and witnesses at the trial of a man who killed 11 congregants at a Pittsburgh synagogue.

The US Justice Department said Hardy Carroll Lloyd is accused of sending threatening social media posts and emails along with comments about the trial of Robert Bowers. 

A jury last week concluded that Bowers should receive the death penalty instead of life in prison without parole for carrying out the attack, dubbed the most antisemitic in US history.

Bowers burst into the Tree of Life synagogue on October 27, 2018 and opened fire with an AR-15 rifle and other guns. Seven people, including five police officers, were also wounded in the attack. 

Lloyd of Follansbee, West Virginia, was also allegedly responsible for stickers placed in predominantly Jewish areas of Pittsburgh directing people to a website containing his threats and antisemitic messages.

Lloyd faces charges of obstruction of the due administration of justice, transmitting threats in interstate and foreign commerce, and witness tampering. 

The charges carry a total maximum punishment of 35 years in prison if convicted. 

In May last year, the Texas Department of Public Safety offered a cash reward of up to $1,000 for information leading to Lloyd's arrest after he allegedly posted a series of comments online threatening to carry a firearm onto the Texas State Capitol grounds and challenge any police officer who tried to "take enforcement actions" against him. 

A statement from the department said Lloyd was a convicted criminal. 

US Attorney William Ihlenfeld said: “Jury trials are a hallmark of the American justice system and attempts to intimidate witnesses or jurors will be met with a strong response.

“The use of hateful threats in an effort to undermine a trial is especially troubling."

The Justice Department described Lloyd as a self-proclaimed "reverend" of a white supremacy movement.