New York City Mayor Bill de Blasio said Friday it was possible someone had “inappropriately accessed” a gas line before a building exploded and caught fire in the East Village of Manhattan Thursday. At least two people remain unaccounted for after the blast, which injured 22 people and left four in critical condition.

“Preliminary investigation suggests this was a gas-related explosion,” De Blasio told reporters at a press conference. The mayor stressed authorities had yet to make an official determination on either the blast’s source or its cause and said it would take about a week for city personnel to clear the area of debris. However, he would not say how officials discovered what de Blasio further described as “a possibility of impropriety” or if a criminal investigation had begun.

Workers have yet to reach the area where contracting work had occurred in the building before the explosion. It was “impossible to say” if a failure of the building’s infrastructure caused the explosion, de Blasio said. New York firefighters have contained fires at the location and city officials have shut down gas flow to the surrounding area.

Inspectors from New York City utility Consolidated Edison arrived at the building for routine inspection and found two individuals performing contract work at the site, possibly without a permit, according to BuzzFeed News. The ConEd employees deemed the work unsatisfactory and did not approve access to the gas line, but the building exploded 15 minutes later. It’s unclear if this incident contributed to the explosion. De Blasio urged city dwellers to call 911 the moment they smell gas.

“We are blessed by the finest first responders anywhere in the world; this is absolutely extraordinary what our first responders did there, it should be a cause for comfort in the midst of this pain,” de Blasio said, according to the Associated Press. “This city knows how to handle adversity. We never welcome it, we know it will come, but we know how to handle it.”