(AP) -- 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.

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 in New Jersey 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.

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

In tapping Pence, a staunch conservative with six terms in Congress, Trump would likely ease GOP leaders' concerns about his own political inexperience and volatile temperament.

Pence 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.

Campaign chair Paul Manafort was among those urging caution in assuming any decision on Thursday, saying on Twitter that Trump's choice "will be made in the near future."

As the day began, it appeared the decision would come quickly. Gingrich, the fiery Republican who helped define the political battles of the 1990s, told The Associated Press that Trump was supposed to let him know something in the afternoon. But by early evening, Gingrich told the AP he had heard nothing from Trump or others in the campaign.

Other Republicans with knowledge of the process said there had been no calls to Pence or Christie, either.

The top contenders have been vetted by a top Washington lawyer and all have spent time with Trump in recent days. But the final decision rests with the candidate, who is known for making decisions more on instinct than other factors -- and for sometimes changing his mind.

Trump was in California Thursday for several fundraisers. His schedule put him at a distance from many of his closest advisers, including Manafort and his three oldest children.

Pence is running for re-election, but Indiana law prevents him from seeking two offices at once. He faces a Friday deadline to withdraw from the governor's race.

The paperwork has been drawn up for him to take that step, according to a Republican, who insisted on anonymity because that person was not authorized to publicly discuss the plans. However, those documents have not been filed.

Christie, in New Jersey, said in an interview with MSNBC: "No matter what phone call (Trump) makes to me today, I will take a deep breath and prepare for tomorrow." The interview came hours before David Samson, whom Christie appointed to chair the powerful Port Authority of New York and New Jersey, pleaded guilty to scheming to get United Airlines to run direct flights between New Jersey and South Carolina where he has a vacation home.

Gingrich was the most visible of the contenders. As speculation about his future swirled, he calmly answered questions about the selection process on Facebook Live, including saying he had told Trump the choice was between having "two pirates on the ticket or a pirate and a relatively stable, more normal person."

Each of Trump's top contenders would add significant political experience to the GOP ticket. Trump has said for weeks that he wanted a running mate who could help him work with Congress.

Beyond their political backgrounds, the finalists bring different strengths to the ticket.

Pence, 57, has deep ties to evangelical Christians and other conservatives, particularly after signing a law last year that critics said would have allowed businesses to deny service to gay people for religious reasons. But the move alienated some moderates in the party.

Trump took notice of Pence during the Indiana primary, noting that the governor had high praise for him despite endorsing one of his rivals.

Gingrich is something of a rabble-rouser who has spent decades in Washington. The 73-year-old would be the oldest candidate ever to become vice president.

Gingrich has been a steadfast Trump defender for months and has become a trusted adviser to the businessman.

So, too, has Christie. The New Jersey governor quickly endorsed Trump after ending his own presidential bid, stunning many of his supporters.

A former U.S. attorney, Christie, 53, is widely seen as one of his party's most talented politicians and has proven himself an effective "attack dog" on the trail. He has also taken on the important role of heading Trump's transition planning.

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AP writers Erica Werner in Washington, and Brian Slodysko in Indianapolis contributed to this report.

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