"Take cover," the French government warned people in Nice via its official terror alert app.
But the alert came almost three hours after police shot the driver of a truck as he plowed through crowds gathered on the waterfront late Thursday to watch a firework display celebrating France's national holiday.
The System to Alert and Inform Populations (SAIP) app, introduced last month, is supposed to provide more timely and informative warnings than the existing nationwide network of sirens and radio messages. The ministry began working on the app after the terrorist attacks in Paris in November 2015, finally putting it into service on June 8.
Thursday night was the first real test of its efficacy, but the government will need to be sure it can rely on it with the country's state of emergency -- initiated after the Paris attacks -- extended for another three months, and the terror alert level expected to remain elevated for some time.
This time around, Twitter provided the more rapid and effective means of communication for officials, with the mayor of Nice, the chief of police and others all taking to the messaging service to provide updates and advice.
The country's Deputy Minister for Digital Affairs, Axelle Lemaire, told those in Nice to "reassure their loved ones by activating the safety check 'safe in Nice' on Facebook" -- a full hour before the Ministry of the Interior's app delivered its message advising people curtly to "take cover" and providing telephone numbers for the public information unit and a victim support group.
The deputy minister's Facebook advice, relayed by the Twitter accounts of many other government officials and departments, drew criticism from some Twitter users for its support of a commercial service.
Facebook activates its Safety Check feature in the wake of certain disasters, natural or man-made, encouraging its users to check in to say they're safe. That way, their contacts on the social network can see they are still alive.
The thousands of people on the seafront Thursday, many of them using their phones to seek information or reassure family and friends, overwhelmed the cellular networks, prompting the Ministry to encourage Twitter users not to jam phone numbers for emergency services, and to telephone only in case of life-threatening emergencies.
As for the Ministry of the Interior's app, it wasn't network congestion but "a technical problem encountered by the contractor who manages the SAIP app," a government source told Les Echos journalist Anaëlle Grondin.
The app contains code developed by Paris-based Deveryware for delivering location-based alerts without the user's location being logged on a central server, an important privacy feature in France.
Representatives of Deveryware had been summoned to the Ministry on Friday afternoon, Grondin reported.
The company did not immediately respond to a request for comment.