An investigation is underway into what led to a massive fire at the LyondellBasell plant in southeast Houston today. The fire broke out around 10am at the Coker unit, which is part of the refining process. There was briefly a shelter-in-place was briefly in effect for residents near the plant, but was quickly lifted. All employees have been accounted for and no injuries were reported. LyondellBasell says air monitoring shows no impact.