Robert Bowers, 48, has been named as the suspect in the mass shooting at a Pittsburgh synagogue that saw 11 people killed and six others injured, including four police officers.
Bowers is a resident of Baldwin, a borough south of Pittsburgh.
He was a user of Gab, a social media site that promotes itself as a bastion of free speech but has become popular with white nationalists, many of whom were blocked by other social media.
According to screenshots of Bower's Gab account, from where most information concerning the alleged synagogue shooter comes, he often wrote anti-Semitic posts.
Bowers' profile description quotes a Bible verse, John 8:44, in which Christ says that members of the Jewish community speaking out against him "belong to their father," or the Devil.
Most biblical scholars believe Christ meant the members of the Jewish community were following Satan's desire to stop a man from speaking the truth.
The profile simply states: "jews [sic] are the children of satan [sic]."
Bowers appears to have associated the US Jewish community with bringing in refugees, a common target of the global far-right.
"[A Jewish refugee organisation, the Hebrew Immigrant Aid Society] likes to bring invaders in that kill our people. I can't sit by and watch my people get slaughtered. Screw your optics, I'm going in," he wrote.
Bowers also posted an image of three pistols. The alleged attacker was reportedly armed with an AR-15 assault rifle and three handguns when he attacked the synagogue.
The alleged assailant also posted images of three pistols that he called his Glock "family".
FBI Agent Bob Jones said Bowers was not known to law enforcement before the attack and it is currently believed the suspect was acting alone, the Associated Press news agency reported.
Hate crimes, not 'terrorism'
Federal authorities have charged Bowers with 29 criminal counts including violence, 11 counts of use of a firearm to commit murder and four counts of obstruction of exercise of religious beliefs resulting in bodily injury to a safety officer and death, which are hate crimes.
"The crimes of violence are based upon the federal civil rights laws prohibiting hate crimes," US Attorney Scott W. Brady and Bob Jones, FBI special agent in charge of Pittsburgh office, said in a statement.
If Bowers is convicted of a hate crime he could face the death penalty.
"Today 11 innocent people were suddenly and viciously murdered during religious services," said Attorney General Jeff Sessions in a statement Saturday. "These alleged crimes are reprehensible and utterly repugnant to the values of this nation."
Hate crimes charges are separate from domestic violence charges under US law.
Most people who face "terrorism" charges in the US are accused to have provided material support to foreign groups designated as "terrorist" organisations by the US government, according to a report by the Center on Law and Security.
The US Constitution affords protection on speech and assembly in the First Amendment, making it difficult - though not impossible - to charge US citizens with terror-related offences that do not involve international actors.
No Trump supporter
US President Donald Trump has said its "incredible" that an anti-Semitic attack could take place.
Trump has faced criticism over his use of divisive rhetoric and support from far-right figures.
A recent string of mail bombs sent to Trump's critics and Democrats, allegedly by suspect Cesar Sayoc, has intensified concern over the president's rhetoric.
The president was quick to say that Bowers was not one of his supporters.
Bowers also posted anti-Trump rhetoric, calling him a "globalist" and not a "nationalist", which Trump recently claimed to be. Bowers used an anti-Semitic slur to say that as long as there Jewish people in the US, the country would never be great.
Hate crimes have increased under the Trump administration. The Anti-Defamation League, a Jewish NGO that tracks anti-Semitism, found that anti-Semitic incidents in the US rose 57 percent in 2017.
It found 1,986 cases of harassment, vandalism or physical assault against Jews and Jewish institutions last year.