MADRID — The head of Portugal’s firefighters’ association said on Wednesday that he believed arsonists had most likely started the fire that killed 64 people last weekend, contradicting a police assessment that attributed the cause to lightning.

The claim is certain to increase concerns about misinformation by the authorities during a fire tragedy that has traumatized the country.

It comes a day after Prime Minister António Costa ordered an investigation into possible flaws in the emergency response, including why motorists were allowed to drive into the fire that they had sought to escape. Most of the fire’s victims died on the road, stranded in their cars.