An explosion at a chain home improvement store in northeastern Romania on Friday injured at least 13 people, four seriously, authorities said.

A mobile intensive care unit was dispatched to the scene in the town of Botosani, in Suceava county, emergency authorities said. Four of the injured are in serious condition and two are intubated, while the others are conscious but have “various traumas and burns” and are receiving medical attention.

Emergency helicopters were alerted, and two ambulances and two fire trucks were sent to the scene to extinguish a fire. A search and rescue mission was underway inside the store. What caused the blast and the identities of those injured were not immediately clear.

The retail company, Dedeman, said in a statement after the incident that at the time of the explosion, which occurred at about 10.30 a.m., an “authorized external company” was working on its Botosani store’s gas supply system and that authorities are working to determine what caused the explosion.