GAINESVILLE, FLa. — Just hours before a pileup that killed 10 people, authorities had closed a stretch of highway smothered in smoke and fog due to a three-vehicle crash that left a passenger seriously injured. Why they reopened it just minutes before the second crash — and who made the call — remained murky Monday, with the Florida Highway Patrol refusing to answer questions as national investigators hinted they might join the inquiry.

An FHP report noted that police closed Interstate 75 about midnight Saturday following the smashup between two SUVS and after drivers called in reporting low visibility. They reopened the roadway at 3:30 a.m. — 15 minutes before at least a dozen cars, six tractor-trailers and a motorhome collided south of Gainesville, pushing some of the cars under the crushing bellies of big rigs, causing others to burst into flames and sending metal shrapnel flying through the air. Eighteen survivors were hospitalized.

FHP spokesmen at the state and local level did not respond to several phone calls and emails Monday from The Associated Press seeking answers to what protocol authorities follow — and whether it had been altered following a similarly deadly crash in 2008 on Interstate 4 between Orlando and Tampa, about 125 miles south of Sunday’s crash. Four people were killed and 38 injured in that crash.

Meanwhile, the National Transportation Safety Board sent investigators to Gainesville on Monday to determine if it would formally join the inquiry into the crash and its causes. Either way, the highway patrol will remain in charge of the investigation, NTSB spokesman Peter Knudson said. “We’ve investigated a number of these pileups and we just want to see if there is any lesson we can learn from this particular series of accidents,” he said. “One of the areas that our investigators will be looking at is the reopening of I-75.”

The pileups happened on both sides of the highway. When rescuers first arrived, the smoke and fog were so thick the only way they could locate victims was by following the sound of their screams and moans from the wreckage, which was strewn for nearly a mile.

Authorities had not released the names of the victims, but did say that four people died in one of the cars and that a “tour bus-like” vehicle also was involved in the pileup. The fog and smoke had cleared enough Monday to reopen all lanes of the roadway.

Steven R. Camps and some friends were driving home hours before dawn Sunday when they were drawn into the massive wreck. “You could hear cars hitting each other. People were crying. People were screaming. It was crazy,” the Gainesville man said hours later. “If I could give you an idea of what it looked like, I would say it looked like the end of the world.”

Before Camps hit the fog bank, a friend who was driving ahead of him in a separate vehicle called to warn of the road conditions. The friend said he had just seen an accident and urged Camps to be careful as he approached the Paynes Prairie area, just south of Gainesville.

A short time later, Camps said, traffic stopped along the northbound lanes. “You couldn’t see anything. People were pulling off the road,” he said. Camps said he began talking about the road conditions to a man in the car stopped next to him when another vehicle hit that man’s car.

The man’s vehicle was crushed under a semi-truck stopped in front of them. Camps said his car was hit twice, but he and another friend were able to jump out. They took cover in the grass on the shoulder of the road. All around them, cars and trucks were on fire, and they could hear explosions as the vehicles burned.

“It was happening on both sides of the road, so there was nowhere to go. It blew my mind,” he said, explaining that the scene “looked like someone was picking up cars and throwing them.”

The fire that is believed to have contributed to poor visibility on the highway had destroyed 62 acres and was still burning Monday. Florida Forest Service spokeswoman Ludie Bond said the fire began Saturday and investigators were still trying to determine whether the blaze had been intentionally set. She said there were no controlled burns in the area and no lightning. No home are threatened by the fire.