WASHINGTON/NEW YORK (Reuters) - The most worrying thing about James Hodgkinson, experts on violent extremism say, is how unremarkable the 66-year-old home inspector from Illinois seemed until he opened fire on Republican lawmakers as they played baseball.
Violent clashes between left-wing and right-wing groups at rallies and protests around the country have increased since the election of President Donald Trump in November.
Experts say detecting and heading off anti-government attacks from people driven by political ideology is increasingly difficult because of the abundance of partisan rancour, particularly on social media.
Hodgkinson wrote a series of strident messages against Trump and other Republicans on his Facebook account.
But so have many other Americans as politics have become more polarized in recent years, particularly since the divisive 2016 presidential election campaign.
None of Hodgkinson's posts suggested he would end up opening fire at a baseball field outside Washington on Wednesday morning. He wounded a top Republican lawmaker, a Congressional aide, a lobbyist, and a Capitol police officer before being shot himself. He later died from his wounds.
In one Facebook post, Hodgkinson wrote: "Trump is a Traitor. Trump Has Destroyed Our Democracy. It's Time to Destroy Trump & Co."
However, there is no evidence so far that he was linked to any radical or violent groups. Like millions of other Americans, he supported Senator Bernie Sanders, an independent who sought the Democratic presidential nomination and condemns violence.
Steve Bongardt, who worked until 2015 as an FBI special agent focussing on threat detection, said traditional counter-terrorism tools such as behavioural profiling and surveillance are less effective because so many otherwise harmless people post virulent messages on social media.
"The problem isn't that behavioural profiles don't work. The problem is the utility of them, because they give us so many false positives," said Bongardt, who now heads The Gyges Group, a security firm.
Mark Pitcavage, a senior research fellow in the Anti-Defamation League's Center on Extremism, said the intensity of emotions on both sides of the political divide could be dangerous.
"When you have people with basically mainstream opinions so worked up that they're willing to commit acts of actual violence, it illustrates in a very stark way how divided our country is right now," Pitcavage said.
A spokesman for the Justice Department said the department is considering a possible statute to target "ideologically motivated crimes of violence" from radical groups or individuals inside the country.
Jerry Boykin, executive vice president of the conservative Family Research Council, which a gunman attacked in 2012 over its opposition to same-sex marriage, said Wednesday's shooting showed that both sides need to "tone down their rhetoric."
"This is an opportunity for a fresh start for everybody in a position of leadership, all the way up to the president," Boykin said.
Most political violence in the United States still comes from right-wing groups, according to Brian Levin, director of the Center for the Study of Hate and Extremism at California State University, San Bernardino.
However, the United States also has a history of violence from left-wing groups such as the Weather Underground, which was active in the 1970s.
It then eased substantially over the past three decades but has risen again in recent years with violence at protests against globalisation, police brutality and the Trump administration, Levin said.
Left-wing extremists "might be the junior varsity, but they're now on the radar screen," he said.
It is too early to say if Hodgkinson's attack was part of a post-election trend of left-wing violence, said J.J. MacNab, a fellow specializing in anti-government extremism at George Washington University's Center for Cyber and Homeland Security.
"We are a cycle-of-violence country. It looks like we may be going into a left-wing phase now, but I'm not sure the violent right-wing is ready to shut up yet," MacNab said.
Reporting by Julia Harte and Dustin Volz in Washington and Dan Trotta in New York; Additional reporting by John Walcott in Washington; Editing by Paul Tait