A Texas chemical plant explosion during Hurricane Harvey underscores the need for industrial plants to rethink emergency preparedness for more serious storms, US investigators said Wednesday.

"Our message is you do have to reassess your worst-case scenario," US Chemical Safety Board director Vanessa Allen Sutherland said in a briefing on the probe of the Arkema chemical plant in Crosby, Texas.

"Plan and plan again," Sutherland said. "Don't be lulled into a false sense of security that it can't or won't happen here."

The Arkema plant, located about 30 miles northeast of Houston, had to be evacuated after it lost power during flooding caused by Harvey, which led to several explosions caused by volatile chemicals that could no longer be refrigerated.

Even after the worst Harvey rains had passed, the plant languished without power and backup generators also lost power due to flooding. Authorities had to evacuate local residents for several days due to the smoke and potential for chemicals in the air.

Arkema officials said Harvey was an unprecedented natural disaster that exceeded all reasonable emergency preparedness.

Investigators said they were still probing what led to the situation and refrained from assessing the company's level of responsibility, or from offering definitive industry recommendations.

But Sutherland said the incident showed how chemical plants must rethink their planning in light of seemingly more severe storms.

"Our key message here is if these types of storms are going to continue... they have to make sure their worst-case scenario is really extrapolating what could happen," she said.

Among the issues still under investigation are whether Arkema sufficiently upgraded its planning after the site was placed by officials in the Texas floodplain in 2007. That designation meant the Arkema site was vulnerable to flooding from 100- or 500-year storms, investigators said.

"They did planning and the question is why wasn't it enough? And what can we learn from that?" Sutherland said.

Sutherland said the agency planned to complete its investigation and issue recommendations in time for the 2018 hurricane season.