A tsunami triggered by a volcanic eruption caught Indonesia by surprise on Saturday night, killing over 370 people and injuring almost 1,500 more.

One of the most horrifying images to emerge from the sudden disaster was a rock band swept off the stage in mid-performance when the deadly wave came ashore.

The death toll mounted with terrible speed on Sunday and Monday as rescuers combed through the wreckage and fresh reports of fatalities poured in. The official count stood at 373 dead on Monday morning, but Indonesia’s disaster agency added that 128 people are still missing. An estimated 11,000 people have been displaced by the disaster.

The tsunami wave hit the coast of southern Sumatra and western Java on Saturday night after a piece of the volcanic Anak Krakatau island collapsed into the sea. Scientists described the event as an “underwater landslide” that came as a deadly surprise because it was not preceded by a massive earthquake. Even tsunami veterans living in the area were stunned by the lack of telltale warning signs before 15-foot waves were upon them.

The tragedy was compounded by damage in Indonesia from a previous tsunami in September, plus earthquakes in July and August. Red Cross delegate Kathy Mueller told NPR on Monday that entire communities in western Java are out of contact and may have suffered uncounted casualties.

“It’s going to take some time before we get a fully clear picture of what the full extent of the damage is,” Mueller predicted.

NPR noted the event will likely prove disastrous for the budding tourist industry in Java, which the Indonesian government hoped to develop into a hotspot on par with Bali. Hundreds of buildings were damaged or destroyed by the tsunami, inflicting a heavy blow on local housing and infrastructure.

Horrifying video of a beachfront rave obliterated by the tsunami went viral on the Internet as the wave simply leaped onstage and devoured a rock band called Seventeen, killing four of its members:

The concert was a holiday event for employees of the state electric company and their families. About 200 people were in attendance when the tsunami struck, many of them children. The electric company stated on Sunday that 29 of its employees and relatives were killed and 13 are still missing.

The Indonesian National Disaster Management Agency issued warnings on Monday that more tsunamis are possible as Anak Krakatau, known fairly literally as the “Child of Krakatoa” because it formed after the fabled eruption of that volcano in the 19th Century, continues to collapse.

Agency spokesman Sutopo Purwo Nugrhoho gave a harsh assessment of Indonesia’s tsunami warning system on Sunday, noting that it currently relies almost entirely on earthquake detection and its network of offshore buoys has not been operational since 2012.

“Vandalism, a limited budget, and technical damage mean there were no tsunami buoys at this time. They need to be rebuilt to strengthen the Indonesian tsunami early warning system,” he advised.