When former special agent Tom O'Connor held a training session for new recruits this month at the FBI¡¯s Virginia headquarters, he turned to a key example to underscore the threat of domestic extremist attacks: the October 2018 mass shooting at a synagogue in Pittsburgh, Pennsylvania.

The shooting, which killed 11 worshippers making it the deadliest attack on the Jewish community in U.S. history, came a little more than a week before congressional elections.

The fact that the alleged gunman was not known to police serves as a stark reminder of the difficulties facing U.S. law enforcement agencies around next Tuesday¡¯s general election, when Republican President Donald Trump will seek to fend off Democratic challenger Joe Biden, former law enforcement officials said.