JAKARTA — No one paid much notice when the Anak Krakatau volcano stirred.
It has been rumbling for months, spewing columns of superheated ash and ribbons of lava that hiss into the currents between Indonesia’s most populated islands.
Beach parties were in full swing under a full moon. Vacationers on Christmas holiday were having a late meal at seaside restaurants on the western tip of the island of Java.
Then — a little over 20 minutes after Anak Krakatau’s latest eruption — the sea rose up without warning, possibly triggered by an underwater landslide shaken loose by the volcanic activity. A wall of water roared ashore, sweeping up everything in its path: boats, tables, people.
A tragic year for Indonesia added more deaths and misery. At least 281 people were counted among the dead by Monday morning — with authorities predicting that the number could rise as disaster teams fanned out in villages and coves along the Sunda Strait, about 60 miles from Jakarta.
Indonesia’s Disaster Management Agency, which announced the death toll, also said more than 1,000 people were injured by the tsunami, which struck late Saturday with deadly stealth from the waters between Java and Sumatra.
Indonesian authorities are always on alert for tsunamis after any seismic activity in one of the world’s most quake-prone regions. But this one caught everyone off guard.
No tsunami alert was raised, and there wasn’t even a big temblor to get the warning system in motion.
The initial speculation by experts, including Indonesia’s meteorology and geophysics agency, was that an undersea landslide pushed the wall of coffee-colored water that raced toward the shore.
Sutopo Purwo Nugroho, spokesman for the disaster agency, said an unusually high tide because of the full moon compounded the surge.
“Data collection is still ongoing. It’s likely that the number of victims and damages will rise,” Nugroho said on Sunday, with the death toll then at 222.
By Monday morning, 57 people were still missing and search efforts expanded along the coast.
David Applegate, associate director for natural hazards at the U.S. Geological Survey, said in a telephone interview Sunday that the tsunami was unusual because it was not among the 90 percent caused by an earthquake. It was most likely caused by volcanic activity that led to a submarine landslide and thus came by surprise, he said.
“This is probably the ultimate no-notice event,” he said.
While it is clear that a large earthquake didn’t trigger the tsunami, a definitive cause probably won’t be determined for a day or so, Applegate said.
“The only question is whether it is related to volcanic activity,” he said.
The tsunami hit with such surprise that the drummer of a local rock band, Seventeen, was starting a song just seconds before the water slammed into the back of the stage at a beach concert. The platform buckled. The band members and their equipment pitched forward as the water swept into the crowd.
At least four people connected to the band were killed, Reuters reported. One crew member, identified by Channel NewsAsia only as Zack, said he survived by holding on to part of the collapsed stage.
“Underwater, I could only pray ‘Jesus Christ, help!’,” Zack said of his struggle in the water. “In the final seconds, I almost ran out of breath.”
The waters ripped houses from foundations, and left cars and boats smashed against palm trees. Thatched bamboo shacks were torn apart. Before daybreak, disaster crews were gathering the dead and creating outdoor morgues piled with orange body bags — scenes that have been repeated many times this year in Indonesia.
Earthquakes and tsunamis have destroyed hundreds of homes and killed and displaced thousands of people this year across the vast archipelago: more than 500 killed on Lombok island in July and more than 2,500 dead in the Central Sulawesi city of Palu in September.
In late October, a Lion Air passenger plane crashed after takeoff from Jakarta, killing all 189 people aboard.
Oystein Lund Andersen, a Norwegian witness who was on a family trip to Anyer Beach, wrote on Facebook that he saw the incoming wave.
“Next wave entered the hotel area where I was staying and downed cars on the road behind it. Managed to evacuate with my family to higher ground through forest paths and villages, where we are taken care of by the locals. Were unharmed, thankfully,” he wrote.
In a statement, Indonesian President Joko Widodo sent his condolences to those affected and said he had dispatched emergency responders. Officials said heavy rescue equipment and emergency soup kitchens were deployed.
At the Vatican, Pope Francis prayed for Indonesians “struck by violent natural calamities.” In Washington, a tweet by President Trump expressed hope for “recovery and healing” amid “unthinkable devastation.”
The disaster came just days before the 14th anniversary of the devastating Indian Ocean quake and tsunami on Dec. 26, 2004, that claimed more than 230,000 lives across the region — with Indonesia hit the hardest.
Igan Sutawijaya, a volcano and geological disaster expert, said the early evidence suggested that an undersea event caused the tsunami.
“My suspicion is that there was a landslide under the sea. Perhaps a trench crumbled,” he told The Washington Post in a phone interview. “It doesn’t make sense that it was caused by the eruption of the Krakatau.”
The volcano’s name translates to “Child of Krakatau,” a volcanic island formed in the 1920s after one of the largest, most devastating eruptions in recorded history occurred at the Krakatau volcano in 1883, which encircled the globe with ash clouds.
In July, the Anak Krakatau volcano put on one of its most violent displays, spewing “lava bombs” as big as trucks.
A report on the website of the Global Volcanism Program, an arm of the Smithsonian Institution, said Anak Krakatau produced ash plumes that reached 200 and 300 feet on Dec. 14 and Dec. 18, respectively. It said “residents were warned to remain outside the 2-km radius zone from the crater.”
Gerry Shih in Beijing, Simon Denyer in Tokyo, and Brian Murphy and Michael Kranish in Washington contributed to this report.