BROWNSVILLE, Texas — At first Yohonny Miratriz heard a deafening roar as he and two friends walked on Sunday morning near the shelter where they had spent the night before.

Then he felt the road tremble beneath him. He turned around to see the gray sport utility vehicle, already on its side, skidding toward him, its horn seemingly stuck in a cacophony of alarm, as the vehicle bore down on him and his friends who had made the long dangerous trek from Venezuela to the Texas border.

“I had time to think, do I jump forward or backward?” Mr. Miratriz recalled on Monday. “Which way would give me a better chance at survival?” He decided to dive head first and felt the sharp sting of the truck’s nose strike his left leg, he said. “At first it didn’t hurt. I was in shock, but then the pain came rushing,” he said a day later, still limping. “How did I survive? I think only God knows.”

A day after the S.U.V., driven by a Brownsville man, George Alvarez, plowed through a group of migrants, killing eight people, much remains uncertain about the tragedy — most important, whether the crash was intentional.

Mr. Alvarez has been charged with eight counts of manslaughter, 10 counts of aggravated assault with a deadly weapon and reckless driving, and remains behind bars, Chief Felix Sauceda of the Brownsville Police said on Monday.

Mr. Alvarez was seen on a video being pinned down on the ground by a group of people outside a homeless shelter that had been housing migrants, most them from Venezuela, after he tried to flee the scene of the crash. His Range Rover S.U.V. struck about 20 people near a bus stop.

Chief Sauceda said the police were investigating reports that Mr. Alvarez had yelled anti-immigration epithets at the group. More criminal charges could be added, he said, if investigators determine that the crash was deliberate.

The police said that they received reports of the crash at around 8:29 a.m. Sunday and that officers found a gruesome scene when they arrived. At least six people were already dead, and four more were badly injured and twisting in pain. Two of the injured later died, Chief Sauceda said.

Investigators said that Mr. Alvarez drove through a red light and appeared to lose control of his vehicle, which flipped on its side and struck the migrants. Chief Sauceda called it “a very tragic scene.”

But for those caught up in the carnage the tragedy was compounded by what it took for the migrants to make it to Texas.

Mr. Miratriz recalled traversing a collection of sharp rocks on a dangerous portion of the Darien Gap, a strip of terrain that connects South and Central America, when he saw a woman lose her footing. She fell to her death, head first, he said.

“I just kept walking, because I knew that if I hesitated, I could fall to my death too,” he said. When he made it to Brownsville with his two friends, he recalled feeling lucky that he had made it when so many perished along the way. And then came Sunday.

One of his good friends Luis Matute, who was in his 30s, died in the crash. The other Johan Quiroz, 40, was severely injured but survived.

The authorities in Brownsville were working with Venezuelan consular officials to identify victims and reunite them with loved ones, Chief Sauceda said.

Mr. Alvarez is a resident of Brownsville who has long been known to the local authorities, Chief Sauceda said, adding that Mr. Alvarez had a record of numerous arrests on charges including driving while intoxicated, burglary of a vehicle, assault and theft, among others.

The police said they were still trying on Monday to determine whether Mr. Alvarez was intoxicated at the time of the crash.

The episode took place outside Ozanam Center, a shelter where a large number of migrants have arrived recently in anticipation of the end of Title 42, a rule imposed early in the coronavirus pandemic by the Trump administration to ease the expulsion of migrants crossing the border. Title 42, which came to be seen as a major impediment to seeking asylum in the U.S., is scheduled to expire this week.

Many Venezuelans have been leaving their native country and seeking asylum to escape poverty and government oppression.

Mr. Alvarez, who has been speaking to investigators in both English and Spanish, has refused to cooperate with investigators, officials said. He has given the police several different names and has not submitted to a breathalyzer test or fingerprinting, they said.

Brownsville, a border town with a majority-Hispanic population in the Rio Grande Valley of Texas, has historically received large influxes of migrants. Located in one of the nation’s poorest regions, the city has recently seen increasing numbers of people crossing the river from Mexico, with as many as 1,500 migrants arriving each day by some estimates.

Victor Maldonado, executive director of the Ozanam Center, said that video footage he reviewed showed that the driver had run a red light before crashing into the crowd of migrants. The center is frequented by migrants who stay for a few days while they work to secure travel elsewhere, Mr. Maldonado said.

“All bodies just started going everywhere, all directions,” he said of the crash.

Michael Eduardo de Aponte Fonseca, who is from Caracas, Venezuela, was still shaken hours after he saw the gray S.U.V. run over several migrants who were standing next to him on Sunday morning. He said that he and other migrants had just eaten breakfast and had trickled out to a bus stop across the street from the shelter. The next thing he knew, he said, the S.U.V. was flying toward them.

“It all happened really fast,” Mr. Fonseca said. “The truck came toward us. It hit a lot of people. All I remember is that a man fell on my legs. I was in shock.”

He ran his hands over his body to make sure he was not injured himself. “Everything became black and white in my brain.”

Cleyver Garcia, his father, who was not with him at the time of the crash, said the two men had gone through an ordeal in leaving their native Venezuela and crossing through the Darien Gap. The two men are headed for Colorado, where they have relatives awaiting them, Mr. Garcia said.

He leaned to hug his son. “I thank God he was not hurt,” he said. “Imagine dying in America, the country we came to to seek safety?”