Experts say ammonium nitrate stored at a Texas fertiliser plant exploded, killing 14 people and injuring hundreds.
The finding was expected and officials had said they were focusing their investigation on the explosive chemical used in many fertilisers, said Rachel Moreno, spokeswoman for the Texas State Fire Marshal's Office.
A spot where the ammonium nitrate was stored is now a 90ft-wide crater, Ms Moreno said.
But the ignition source for the explosive chemical remained undetermined.
Findings on the cause of the blast on April 17 on the outskirts of the small town of West initially had been expected on Friday. But the probe will take one to two extra weeks to complete, with dozens of investigators combing through plant wreckage and the adjoining wrecked neighbourhood, Ms Moreno said.
Government emergency officials have begun offering shelter for West residents whose homes were destroyed or severely damaged. About 70 properties were affected.
The Federal Emergency Management Administration said the transitional sheltering assistance was requested by Texas state officials. It will allow those whose homes were left uninhabitable by the blast to stay for a limited time in a hotel or motel at government expense.
Meals, telephone calls and other incidental charges are not covered and people are responsible for any costs above those authorised, said an administration statement.