PARIS — Churches around France held Sunday services honoring three people killed in an Islamic extremist attack at Notre Dame Basilica in the city of Nice that pushed the country into high security alert, while police questioned six suspects in the case.

Nice Archbishop Andre Marceau was preparing for a special nighttime service in the basilica to purify it following Thursday’s fatal knife attack, and then to pay homage to the victims and to mark All Saints’ Day, when many Christians around the world honor the dead.

Priests in the Saint-Sulpice Church in Paris and elsewhere in France mentioned the attack during their All Saints’ services, which were exceptionally allowed to go ahead despite a new monthlong virus lockdown that started Friday in France. Riot police or other security forces were stationed at some prominent religious sites.

Authorities have labeled the church killings an act of Islamist terrorism. They took place amid global tensions around cartoons published by a French newspaper mocking Muhammad, which deeply offend Muslims. French imams and other Muslims were among the many who denounced the Nice attack as having nothing to do with their faith, and called for calm.

The three victims were 55-year-old church warden Vincent Loques, a father of two described by parishioners and nearby merchants as “nice to everyone”; churchgoer Nadine Devillers, 60; and Brazilian-born Simone Barreto Silva, 44, Brazilian media said Silva, a mother of three, moved to France to join a dance group led by her sister, worked in elder care and dreamed of traveling the world in a food truck.

Investigators in France, Tunisia and Italy are trying to determine the motive of chief suspect Ibrahim Issaoui, a 21-year-old Tunisian who transited through Italy last month en route to France, and whether he acted alone.

Issaoui is in critical condition in a French hospital after being wounded by police during his arrest and hasn’t yet been questioned, according to a judicial official.

Five other people were also in custody Sunday after being detained in Nice and the nearby town of Grasse, the official said. They are between 25 and 63-years-old and were spotted on video surveillance or detained in homes searched by police as part of the investigation, said the official, who was not authorized to be publicly named according to judicial policy.

Their connection to the attack remains unclear. A previously unknown Tunisian extremist group claimed responsibility for Thursday’s attack. Tunisian and French authorities are investigating whether the claim is legitimate.