Donald TrumpDonald TrumpHouse votes to condemn Chinese government over Hong Kong Former Vice President Walter Mondale dies at age 93 White House readies for Chauvin verdict MORE was repeatedly booed by the GOP presidential debate audience Saturday in South Carolina when he criticized former President George W. Bush for failing to keep America safe on Sept. 11, 2001. 

"How did he keep us safe when the World Trade Center went down?" the party front-runner said to boos.
"I lost hundreds of friends, the World Trade Center went down during the reign of George Bush. He kept us safe? That is not safe." 
"The World Trade Center came down because Bill ClintonWilliam (Bill) Jefferson ClintonObama, Clinton reflect on Mondale's legacy Polls suggest House Democrats will buck midterm curse and add to their ranks Democratic Rep. Mondaire Jones calls on Breyer to retire MORE didn't kill Osama bin Laden when he had the chance to kill him," Rubio said, referncing the 1993 attack.
In an audio recording of Clinton from the day before the 2001 attacks that wasn't released until 2014, he admitted that he had the chance to kill bin Laden while in office but decided against the strike out of concern for hundreds of civilian casualties.  
Trump responded to Rubio's barb by arguing that Bush had the chance to kill bin Laden as well and similarly decided against it. 
The comments came minutes after Trump first brought up the 9/11 attacks in a barb with Bush's brother, White House hopeful Jeb Bush.