WATCH: Tourist Boat In Colombia Sinks, 9 Dead, Dozens Missing

A tourist boat carrying over 150 people sank Sunday on a reservoir near the Colombian city of Medellin, local authorities confirmed. Authorities said nine people were dead and over 28 people were missing after the boat sank.

The four-storeyed boat, El Almirante, went down near the Guatape reservoir and videos on social media showed the turquoise and yellow-trimmed recreational boat rocking back and forth while people began to climb down from the fourth storey as the vessel began to sink.

The reservoir, situated a short distance from Medellin in Antioquia department (geographical division like a state) in northwest Colombia, has been a popular tourist spot with paddle boarders and kayakers. And it was packed with tourists on Sunday as this weekend was long one, the Associated Press (AP) reported.

Margarita Moncada, the head of the disaster response agency in Antioquia department, told reporters while the rescue operations were ongoing, that 99 people were rescued and about 40 people managed on their own to swim back to the shore and were healthy.

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Authorities confirmed that the injured were being treated in a local hospital. Rescue operations were immediately undertaken, including deploymnt of firefighters and air force pilots. The Colombian air force said it was also conducting a helicopter rescue operation and specialized divers were searching for the missing. The country's President Juan Manuel Santos was present at the scene and said relief agencies were assisting in the rescue operations, BBC News reported.

The state government of Antioquia posted on its Twitter account, saying seven people were discharged after being treated at the local hospital while 24 among them were still under careful watch.

A local restaurant owner, Daniel Giraldo, told AP he ran to the scene as soon as he heard ambulance sirens, however, the boat was almost submerged by then. "It sank in a matter of four minutes," he said.

Eyewitness Louisa Murphy told BBC: "We saw things flying off the side of the boat. And within, I think, probably about 20 seconds the boat had sort of sunk with just the top deck visible."

The cause behind the sinking of the boat was not clear immediately, however, a local television network, Teleantioquia, reported the boat might have sunk because it was overcrowded. The television network specified the vessel had a maximum capacity of 150 passengers, but was carrying 170 passengers on Sunday. Some passengers who were rescued told AP they were not given life vests. Two residents of Guatape, the reservoir where the boat sank, told Blu Radio in Colombia that the El Almirante had sunk earlier, about three months ago, while it was tied at the dock. "They fixed it and it kept working normally," one of the residents told the radio network, the Guardian reported.

"As governor of Antioquia I regret this untimely tragedy," Antioquia's Governor Luis Perez Gutierrez posted on Twitter, referring to the incident in the reservoir near the Colombian city of Medellin.

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A U.S. State Department official told NBC News that they were unaware if any U.S. citizens had been affected or injured in the incident.