A tourist boat packed with more than 150 passengers capsized Sunday on a reservoir near the Colombian city of Medellin, leaving at least six people dead and 15 others missing.

Rescuers including firefighters from nearby cities and air force pilots were searching for survivors at the reservoir where the El Almirante sank with more than 150 passengers on board.

A flotilla of recreational boats and jet skis rushed to the scene, pulling people from the boat as it went down and avoiding an even deadlier tragedy. It is not clear how the boat capsized.

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“Nobody really knows what happened,” said Colombian President Juan Manuel Santos, adding that naval officials were brought in to carry out an investigation.

Dramatic videos circulating on social media show the party boat rocking back and forth as people crawled down from a fourth-floor roof as it sank in a matter of minutes. Survivors described hearing a loud explosion near the men's bathroom that knocked out power a few minutes after the boat began its cruise around the giant lake.

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Of the 134 people who survived the crash, three remain hospitalized but are out of danger, said Margarita Moncada, the head of the disaster relief agency in Antioquia state.

A group of 25 scuba divers had to suspend their search overnight due to a lightning storm, but they resumed their work on Monday morning.

The Associated Press contributed to this report.

