6 People Dead, 15 People Missing After Tourist Boat Sinks: 'Nobody Really Knows What Happened'

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Authorities in Colombia are searching for 15 people who remain unaccounted for after a tourist boat capsized in a reservoir on Sunday afternoon.

Read: Whale Watching Boat Sinks, Leaving at Least 5 Dead including men aged 18 and 76

Six passengers perished and three remain hospitalized after the boat sank in Guatape, near Medellin.

The boat named “El Almirante,” which translates to “The Admiral,” was carrying more than 150 people, as it was going through a reservoir between Medellin and Guatape during a holiday weekend in the country.

The multi-decker boat was filmed rocking violently back and forth as it began to flip over.

Authorities still do not know what caused the boat to sink. According to Colombian President Juan Manuel Santos, the boat was not overcrowded.

“Nobody really knows what happened,” Santos said.

“Those on the first and second decks sank immediately,” survivor Lorena Salazar told local media. “All we could do was scream and call for help. It was completely chaotic.”

Read: 3 Teens Rescued From Capsized Boat Miles From Shore: 'They Were Lucky They Had a Cell Signal'

Since there was no passenger manifest listing who was on the boat, so authorities are working with survivors and family members to determine the identities of those on board.

On Sunday night, scuba divers and rescue crews had to suspend their search due to a thunder and lightning storm in the area. The search resumed early Monday morning.

Watch: Puppy That Fell Off Boat Swims 6 Miles Back to Shore to Reunite With Family