(Reuters) - No drugs or alcohol were found in the system of the man accused of vehicular homicide in the crash of a Tennessee elementary school bus that killed six children and critically injured several more, police said on Wednesday.

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Johnthony Walker, 24, was driving the bus full of students home from Chattanooga’s Woodmore Elementary School on Monday when it veered off a road, flipped on its side and smashed into a tree and telephone poll, according to a police affidavit.

Officials said five children were killed initially: a kindergartner, a first-grader and three fourth-grade students. Police said on Wednesday evening that a sixth child had died.

Walker was driving on a narrow, winding road at well above the speed limit of 30 miles per hour (48 kph) when he lost control, leaving the bright yellow bus mangled and nearly severed in two, the affidavit said.

Walker has been charged with five counts of vehicular homicide, reckless endangerment and reckless driving.

National Transportation Safety Board Chairman Christopher Hart told reporters on Wednesday there had been no fatal crashes on that stretch of road over the past three years.

Hart added that the road was not on the bus’ designated route, but he did not provide an explanation why the bus was there.

Six students were rushed to the hospital in critical condition immediately after the crash and another six were hospitalized with less severe injuries before being released, hospital officials said. Nineteen other students were treated and released within hours of the crash, hospital officials said.

Six children remained hospitalized, but the hospital did not provide their current conditions, citing privacy, Chattanooga Police Sergeant Austin Garrett told reporters.

Garrett said there was no indication that Walker intentionally tried to harm the children.

Walker’s mother, Gwenevere Cook, told CNN that her son tried to rescue students from the bus after the crash but blood and the children’s limp bodies made it difficult.

No children were thrown from the bus during the crash, Garrett said. The bus was not equipped with seat belts but had two cameras on board as well as an engine chip that could contain information, according to the NTSB.

The mother of an 8-year-old child killed in the crash filed a lawsuit against Walker and bus operator Durham School Services alleging negligence, according to documents uploaded online by local broadcaster WTVC.