A devastating tsunami took Indonesians by surprise because an early warning system – designed to detect changes in wave height in coastal areas near an active volcano – hasn't worked since 2012, government officials said.

The system uses a series of 22 buoys, tidal gauges and seismographs to alert a network of sirens in coastal areas vulnerable to tsunamis. But Sutopo Purwo Nugroho, spokesman for Indonesia's National Board for Disaster Management, said the system has been crippled by vandalism, damage and neglect.

"The absence of this system caused the failure to detect the tsunami in advance," he said.

Indonesian authorities updated the death toll to at least 373 people Monday, with 1,459 people injured. And 128 people remain missing, as search-and-rescue crews deal with damaged roads and bridges as they attempt to reach hard-hit areas with heavy equipment.

About 11,453 people were evacuated from coastal areas along the Sunda Strait, which separates the islands of Sumatra and Java, but the government said about half of those have been able to return.

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The U.S. State Department was not aware of any American citizens affected, spokesman Robert Palladino said. The Indonesian Medical Association said most victims it's treating were domestic tourists, who had swelled the beachfront villas and hotels during the long weekend ahead of Christmas.

The tsunami followed an eruption and landslide on Anak Krakatau, the island known as “Child of Krakatoa” because it was created in 1927 from one of the world's most active volcanoes.

The 1,000-foot-high volcano has been erupting since June and did so again about 24 minutes before the tsunami, said Indonesia’s Meteorology and Geophysics agency.

But Saturday's eruption came without the massive earthquake that has preceded previous tsunamis, giving residents little warning of the wall of water that would come.

It was the second deadly tsunami to hit Indonesia this year. The September tsunami that hit Sulawesi island was triggered by a powerful earthquake, giving residents a brief warning before the waves struck.

But on Saturday night, the tsunami came so suddenly that revelers at a Tanjung Lesung resort were swept into the sea as they attended a beachfront concert.

More:Video shows the haunting moment tsunami sweeps away Indonesian band

Dramatic video posted on social media showed the Indonesian pop band Seventeen performing under a tent at a concert for employees of a state-owned electricity company. Dozens of people sat at tables while others swayed to the music near the stage as strobe lights flashed and theatrical smoke was released. A child could also be seen wandering through the crowd.

Seconds later, with the drummer pounding just as the next song was about to begin, the stage suddenly heaved forward and buckled under the force of the water, tossing the band and its equipment into the audience.

The group released a statement saying their bass player, guitarist and road manager were killed, while two other band members and the wife of one of the performers were missing. On Monday, five more bodies were recovered around the hotel, including a little boy.

“The tide rose to the surface and dragged all the people on site,” the band’s statement said. “Unfortunately, when the current receded, our members were unable to save themselves while some did not find a place to hold on.”

The worst-affected area was the Pandeglang region of Java’s Banten province, which encompasses Ujung Kulon National Park and popular beaches, the Indonesian disaster agency said.

Indonesian President Joko “Jokowi” Widodo arrived at the disaster area by helicopter on Monday. A day earlier, he expressed his sympathy and ordered government agencies to respond quickly to the disaster.

“My deep condolences to the victims in Banten and Lumpung provinces,” he said Sunday. “Hopefully, those who are left have patience.”

In the city of Bandar Lampung on Sumatra island, hundreds of residents took refuge at the governor’s office, while at the popular resort area of Anyer beach on Java, some survivors wandered in the debris.

Many of the affected areas are popular weekend getaways for residents of Jakarta, Indonesia’s capital, but foreigners were also visiting the area over the long holiday weekend.

Oystein Lund Andersen, a Norwegian diplomat and volcano photographer, was shooting pictures of the eruption from across the Sunda Strait on Saturday.

"Earlier in the evening, there was quite heavy eruption activity," he told the BBC. "But just prior to the waves hitting the beach, there was no activity at all. It was just dark out there.

"And suddenly I saw this wave coming, and I had to run."

There were actually two waves, Andersen said. The first came over the beach and hit about 50 feet behind where he was standing. But the second passed his hotel, sweeping cars and debris with it as it surged further inland.

Geologists speculated that the erupting volcano caused massive landslides either into or under the water.

Gegar Prasetya, co-founder of the Tsunami Research Center Indonesia, said Saturday’s tsunami was likely caused by a flank collapse — when a big section of a volcano’s slope gives way.

“Actually, the tsunami was not really big, only 1 meter (3.3 feet),” said Prasetya, who has studied Krakatoa. “The problem is people always tend to build everything close to the shoreline.”

Indonesia, the world's fourth largest nation, is made up of thousands of volcanic islands along the western edge of the Pacific Ocean's "Ring of Fire." And the Krakatoa formation is one of the most active and destructive volcanoes in the world.

An 1883 eruption triggered tsunamis that killed more than 30,000 people, causing the volcano to collapse in on itself. The 1927 eruption created a new volcanic island, Anak Krakatau, that continues to grow each year.

A powerful earthquake on the island of Lombok killed 505 people in August. The tsunami and earthquake that hit Sulawesi in September killed more than 2,100, while thousands more are believed to still be buried in neighborhoods swallowed by a quake phenomenon known as liquefaction.

Saturday’s tsunami also rekindled memories of the massive magnitude 9.1 earthquake that hit Indonesia on Dec. 26, 2004. It spawned a giant tsunami off Sumatra island, killing more than 230,000 people in a dozen countries — the majority in Indonesia.



Contributing: The Associated Press