A day after a natural gas explosion rocked a Steveston apartment complex, the investigation into what caused the blast is expanding.

The investigation will now include Technical Safety BC, which oversees the installation and operation of technical equipment in the province.

At the time of the blast, workers were in the unit where it happened servicing a natural gas fireplace.

"Technical Safety BC has sent a Gas Safety Officer and a member of our Incident Investigations team to look into this further, and understand the root causes and circumstances that led to this event," the agency said in a statement.

"We don’t want to speculate on the cause of the incident and will provide further information after our investigation is complete. To our knowledge, the gas work being done in the building was being done by a licensed contractor under the appropriate permits."

The explosion caused significant damage to four units in the building, moving interior walls and forcing residents out for an indefinite period of time.

Ralf Exner was outside at the time of the explosion and says the force of it felt like someone shoving him.

"I heard a loud explosion and I looked up and I saw those roof soffits blowing out and insulation coming down," he said. "And I actually sat here and I felt the shockwave from it."

Technical Safety BC says it will be working with the City of Richmond and the fire department as it investigates what led to the explosion.