Republican White House hopefuls Donald Trump and Jeb Bush locked horns after frontrunner Trump implied that former president George W. Bush shared some blame for the September 11, 2001 attacks.
"When you talk about George Bush, I mean, say what you want, the World Trade Center came down during his time," Trump said on Bloomberg TV, in just his latest controversial remarks.
George W. Bush was president during the attacks that killed about 3,000 people.
Jeb Bush, whose campaign for the White House has stuttered after a string of gaffes, branded Trump "pathetic."
"We were attacked & my brother kept us safe," Bush hit back on Twitter, the latest episode in a long-running and mounting feud between the pair.
Ratcheting up the row a notch further, Trump tweeted back: "You're pathetic for saying nothing happened during your brother's term when the World Trade Center was attacked and came down."
Trump, who has surged into the lead for the Republican nomination at the expense of Bush, followed up with another barb at his campaign rival, tweeting: "Like it or not, our country needs more energy and spirit than you can provide!"