• The attack in Nice occurred on France’s national holiday — 227 years since the storming of the Bastille prison, a pivotal moment in the French Revolution — and in one of France’s most populous cities, during peak vacation season.

What We Don’t Know

• Who committed the attacks, and why. The newspaper Nice Matin reported early Friday that he was a 31-year-old Frenchman of Tunisian origin, but officials have not confirmed that.

• The extent and adequacy of the preparations that French officials had put in place to handle large crowds during the Bastille Day celebrations, especially in cities outside Paris, the capital. Extensive security, including the hiring of private security agents, had been put in place for the recent European Championship soccer tournament.

• Whether France’s intelligence and security agencies had received any hints of the danger. On July 5, a parliamentary inquiry examining last year’s attacks found widespread failures in the collection and analysis of information that could have helped prevent those assaults.