Authorities demolished two boiler towers at a decommissioned thermal power plant in the southeastern city of Ulsan that had hindered efforts to find people trapped underneath debris after a nearby third structure collapsed Thursday.

The 63-meter towers on either side of the debris, which had been on the verge of collapse, fell almost simultaneously following a scheduled detonation at noon on Tuesday.

The shock wave could be felt up to 300 meters away. Fragments were scattered, and particles spread like smoke.

Once authorities complete follow-up safety measures within two to three hours of the blast, they are expected to exert full efforts in the search for four missing workers—two of whom are suspected to be dead.

Heavy equipment, including cranes, will be deployed at the scene.

Of the seven people who were initially trapped underneath, three have been found and confirmed dead.