The vessel was packed with about 160 passengers for holiday weekend and survivors say they heard a loud blast near men's bathroom.

A tourist boat packed with about 160 passengers for the holiday weekend capsized on Sunday on a reservoir near the Colombian city of Medellin, leaving at least six people dead and 31 missing, officials said.

Rescuers including firefighters from nearby cities and air force pilots searched for survivors at the Guatape reservoir where El Almirante ferry sank. 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.

Dramatic videos circulating on social media show the turquoise and yellow trimmed party boat rocking back and forth as people crawled down from a fourth-floor roof as it sank into the water in a matter of a few minutes.

Loud explosion?

Survivors described hearing a loud explosion near the men’s bathroom that knocked out the power a few minutes after it began its cruise around the giant lake. As water flooded on board, pressure built and people were sucked under by the sinking ship.

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

Margarita Moncada, the head of the disaster response agency in Antioquia state, said that according to a preliminary report 99 people were rescued and another 40 managed to find a way to shore on their own. Speaking to reporters from the reservoir, she said nine people had been killed and around 28 are still missing.

Santos takes stock

But later Sunday President Juan Manuel Santos arrived in Guatape and said 122 people were either rescued or found their way to shore and were in mostly good condition. Six had died and another 31 were missing, he said. The discrepancies in the number of fatalities could not be immediately reconciled.

It’s unclear what caused the boat to sink.

Some people who witnessed the tragedy from the nearby shore said the boat appeared to be overloaded but Santos said it was sailing well below capacity. None of the passengers were wearing a life vest. Complicating the search, there wasn’t even a passenger list.

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

Carlos Espinosa, an independent journalist from Guatape, said about a month ago townspeople awoke to find the El Almirante filled with water and sinking at its dock, suggesting that perhaps the vessel wasn’t ready to return to the water.

“What makes you angry is there are no controls by the government,” he said.

Silence sets in festive town

As night fell, the usually festive town was silent as people began to register the magnitude of the loss. Among those huddled under the rain near the port looking for information about loved ones was Alberto Villegas, who was separated from a cousin and uncle in the mad rush to abandon the sinking ship.

“All we ask is that they don’t give up the search,” said Mr. Villegas.

Authorities were at a loss to say exactly how many people were on the boat and asked passengers or their loved ones to report to a rescue center hastily set up along the shore. They also made a call for scuba divers to assist with the search.

The reservoir surrounding the soaring rocky outcrop of El Penol is a popular weekend destination a little more than an hour from Medellin. It was especially busy on Sunday as Colombians celebrated a long holiday weekend.