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

Trump had planned to hold his first event with his yet-to-be-named running mate this 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 Thursday in the French resort city of Nice when a truck 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 strengthen alliances, including with NATO, to ferret out terrorism and prevent future attacks. Clinton said she would intensify efforts to put together a 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.

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.

Pence, a staunch conservative who served six terms in Congress, is seen as a running mate who would have the backing of GOP leaders and ease some of their concerns about Trump’s political inexperience and volatile temperament.

He has influential allies in Trump’s inner circle. But some of Trump’s children, who have been closely advising their father, are said to favor different candidates.