Speaking with NBC, Clinton called it a terrorist attack and noted that ISIS - by that time - had taken credit for it.
Police and security officials increased security levels in NY, after a big explosion ripped through the Chelsea neighborhood in Manhattan, injuring 29 people.
Hillary Clinton's campaign is aggressively outworking Donald Trump in battleground Pennsylvania, a state the billionaire businessman can scarcely afford to lose and still hope to become president.
"Hillary Clinton's comments today accusing Mr. Trump of treason are not only beyond the pale, it's also an attempt to distract from her awful record on ISIS", said Jason Miller, spokesman for the campaign.
In his Fox News interview, Trump was asked about his conclusion before investigators had announced their findings that a bomb had been detonated.
Trump is often loose with his language in describing what has taken place, and couples it at times with bragging about his own calls for toughness.
On Sunday, New York Gov. Andrew Cuomo, D, said there was "no evidence of an worldwide terrorism connection" to the incident in the Chelsea neighborhood of Manhattan, where an "intentional" blast apparently was caused by an improvised explosive device on Saturday. On Sunday, Islamic State claimed responsibility, calling the man "a soldier", and the Federal Bureau of Investigation said it was investigating the attack as a potential act of terrorism. It is not overstating the case to say that both candidates are hated, but at this stage Republicans could be forgiven for resorting to that all-purpose adage in which those who invoke it insert their epithet of choice - which in this case would be something like: "he might be a f-- wit, but he's our f-- wit". "But we're not going to go after an entire religion and give ISIS exactly what it wants".
In another time and with another candidate, Trump's freakish "Obama is not/is an American" nonsense and his reckless return on Friday to language that seems to invite a gun nut to go for Clinton would have sealed the deal for a runaway Democratic win.
He later stated his comments had been intended as a joke, but repeated a similar sentiment at Friday's (16 September) rally in Miami when he told the crowd: "I think that her bodyguards should drop all weapons".
CLINTON: Well, Jennifer, I don't want to speculate but here's what we know. A pair of "Love Trumps Hate" signs flanked Clinton on stage, and it was clear that she hopes that Trump's incendiary speeches will be enough to bring around the young voters now tempted to vote for a third party.
Clinton talks and tweets so incessantly about Trump it's really getting hard to remember what she's proposing herself anymore.
"Yet despite this, I remain confidence that America's best days are ahead of us", Clinton said, pointing to the "inspiring young people I meet every day". Her campaign and outside groups helping her have spent about $14 million on general election TV and radio ads through this week, according to Kantar Media's political ad tracker.
After the shooting, on CNN, he said: "Whenever there's a tragedy, everything goes up, my numbers go way up because we have no strength in this country, we have weak, sad politicians".
"In other breaking news, the world is round, not flat", he said.