Mohamed Lahouaiej Bouhlel was behind the wheel of a truck that barreled into Bastille Day revelers and claimed 84 lives. (Thomas Johnson/The Washington Post)

With the arrest of three more suspects Sunday, French authorities are widening their investigation into the Bastille Day truck attack that killed 84 people in this French Riviera city to include potential accomplices of the man the Islamic State described as one of its warriors.

French authorities arrested an Albanian man and woman who they believed supplied weapons to the 31-year-old attacker, Mohamed Lahouaiej Bouhlel. Authorities also said Bouhlel, who has been described by family and friends as a troubled loner, had sent a text message saying “Bring more weapons” to another suspected accomplice and had visited the scene of the attack days before the carnage.

Late Sunday night, authorities announced that they had detained the man believed to be the recipient of the text message. Seven others had been detained earlier in connection with the Thursday attack. They included Bouhlel’s ex-wife, but she was reportedly released from custody Sunday morning, according to authorities speaking to local media.

Bouhlel, 31, was a “soldier of the Islamic State,” the militant group declared Saturday. French authorities said the Tunisian-born attacker was inspired by terrorist organizations.

Twelve victims have not been identified, France’s minister of health, Marisol Touraine, told reporters Sunday. A 6-month-old child was among 18 people still in critical condition in the hospital.

The number of wounded rose to 256, according to the French prosecutor’s office, with 85 still hospitalized.

Bouhlel’s text message was sent at 10:27 p.m. Thursday, roughly a half-hour before the attack on the landmark Promenade des Anglais, where tens of thousands had gathered to watch a Bastille Day fireworks display. The phone Bouhlel used, police said, was found inside the rented 19-ton refrigerated truck after he mowed down people for more than a mile and was killed in a shootout with police.

French authorities on Sunday also provided more details that suggested the attack was carefully premeditated.

Two days before the violence, security-camera footage showed Bouhlel driving the truck on the Promenade des Anglais, apparently staking it out, Jean-Michel Decugis, head of the judicial police department, told local media. He added that Bouhlel also emptied out his bank account, according to one of the suspects in custody. He was supposed to have returned the truck to the rental company the day before the attack.

Investigators are also continuing to look into Bouhlel’s possible path to radicalization. It remains unclear whether the Islamic State orchestrated the attack, inspired Bouhlel and was taking responsibility, or was seeking publicity despite having no direct influence.

Thursday’s attack was the third major terrorist assault in France in the past 18 months. In January 2015, militants targeted the offices of the Charlie Hebdo satirical newspaper in Paris, and on Nov. 13, gunmen launched a wave of attacks in the city, focusing on the Bataclan concert hall. A huge outpouring of solidarity and resilience followed those attacks. But in the aftermath of the carnage here, anger and resentment at the government have erupted over what many French describe as a failure of security to protect the Bastille Day celebrations.

On Sunday, though, a sense of normality returned. Thousands of sunbathers flocked to the beaches of the Promenade des Anglais, sipping wine under blue umbrellas or windsurfing on the blue waters of the Côte D’Azur.

Annabell Van den Berghe contributed to this report.