WASHINGTON — Donald Trump abruptly postponed plans to announce his vice presidential pick following a day of rampant speculation, citing the “horrible attack” in Nice, France, that left scores dead.

Trump had planned to hold his first event with his yet-to-be-named running mate Friday morning in New York. He announced the change of plans Thursday evening on Twitter.

The stunning announcement raised questions about the status of Trump’s selection process. Indiana Gov. Mike Pence had emerged as a late favorite for the job, though Trump said he had not finalized the pick and advisers cautioned he could change his mind.

“I haven’t made my final, final decision,” Trump said on Fox News Channel. He said that while his running mate selection would “absolutely not” be changed by the France attack, he did not feel it was appropriate to hold a news conference in its aftermath.

Dozens of people were killed late Thursday in the French resort city of Nice when a truck drove onto a sidewalk and plowed through a crowd of Bastille Day revelers who’d gathered to watch fireworks. Trump told Fox News after the attack that if he’s elected president he would ask Congress for a declaration of war on the Islamic State group.

Democrat Hillary Clinton, also appearing on Fox, said the U.S. needs to “stand strongly” with France and said she would intensify efforts to put together a more effective coalition against terrorism.

In addition to Pence, Trump’s vice presidential shortlist included former House Speaker Newt Gingrich and New Jersey Gov. Chris Christie, according to people familiar with the candidate’s thinking.

After spending much of Thursday in Indianapolis, Pence flew to New York late in the day, according to a Republican familiar with the process. Indianapolis television station WTHR posted a video showing Pence arriving at a private airport outside New York early Thursday evening.

Trump did not say when he planned to announce his running mate. He’s up against a clock: The Republican convention kicks off in Cleveland Monday.

Top party officials are already in Cleveland, grappling with a rules fight that could increase the odds of nationally televised clashes at the convention. Late Thursday, a committee at the Republican National Convention defeated an effort by conservatives who want to let delegates vote for any presidential candidate they’d like. Conservatives hoped that would lead to delegates blocking Trump’s nomination.