DALLAS — The federal agency that investigates industrial accidents offered muted criticism Thursday of the Houston-area chemical plant that flooded during Hurricane Harvey and partially exploded.

Crews for Arkema Inc. worked "to the best of their ability" to keep equipment that cooled and stabilized its organic peroxides from losing power as six feet of water engulfed the plant last August, said Mark Wingard, lead investigator for the U.S. Chemical Safety Board, during a news conference in Houston.

Arkema's emergency plans, however, hadn't anticipated the damage that high flood waters could cause its safety systems. Employees at the plant in Crosby, just outside Houston, told investigators they had only seen a foot or two of flood water in the past.

As a result, Arkema didn't consider flooding of safety systems "a credible risk," even though the plant was inside government-designated flood zones and its insurance company warned in 2016 that it was at risk, according to the safety board's 154-page report.

"Once the rain started falling and once Harvey was here, it was almost too late," Wingard said. "Industry relying on personal information (from employees) about how high your flood waters could get might not prepare you for the worst-case scenario."

The agency has no authority to fine or cite companies, but instead makes safety recommendations. In its report, it urged companies and industry groups to create guidance that helps facilities guard against flooding and severe weather events, as the United Kingdom does. What the U.S. has now wouldn't have helped Arkema, the safety board said.

In a written statement Thursday, Arkema officials noted the safety board had credited it for having safeguards that likely would've worked if employees had more time to relocate chemicals to higher ground. Arkema, whose parent company is based in France, has previously said no one could've foreseen Harvey's unprecedented flooding, but it had taken multiple precautions and continues to study improvements.

Arkema, meanwhile, still faces an investigation by local county prosecutors, who said in a statement Thursday that they plan to present evidence to a grand jury in the coming weeks.

After Arkema's plant lost its primary and backup sources of power, its organic peroxides began heating and decomposing. The compounds, used in a range of products from plastics to paints, eventually caught fire and partially exploded, sending plumes of smoke skyward.

First responders and neighbors said they were sickened after the incident and blamed Arkema's releases. Testing performed on soil, water and ash samples detected amounts of toxic substances, according to lawsuits they filed against Arkema.

The safety board said it was "impossible" to analyze or draw conclusions about the health effects from the exposure, in part, because of the type of air quality testing the U.S. Environmental Protection Agency used. Safety board member Kristen Kulinowski declined to elaborate during the news conference.

The safety board also renewed its call for the EPA to include reactive chemicals like organic peroxides on the list of compounds that require accident planning.