Rescue operations have resumed at the Ulsan thermal power plant after two nearby boiler towers were demolished, removing safety obstacles that had delayed the search.

Authorities said teams resumed work at 3:40 p.m. Tuesday and that cutting equipment had been deployed to reach the areas where workers are believed to be trapped.

Four workers remain unaccounted for—two are presumed dead based on their detected locations, and two others are still missing.

Rescue dogs and detection devices are being used as crews expand the search radius around the debris of the collapsed No. 5 boiler tower.

The adjacent No. 4 and No. 6 towers, which posed a risk of secondary collapse, were demolished at noon, allowing heavy cranes and rescue specialists to enter the site safely.

The 63-meter structure collapsed last Thursday, killing three workers and trapping the rest during dismantling work at the Korea East-West Power Company’s Ulsan facility.