Government authorities in Guyana said on Monday night that the bodies of at least 27 people, including four children, had been found after a ferry thought to be overloaded capsized over the weekend in waters off the South American nation. The officials warned that the death toll could climb, because dozens of others are believed to be missing.

The ferry capsized late Saturday night while on a regularly scheduled route from Georgetown, the capital, to the remote gold-mining outpost of Port Kaituma, in northwestern Guyana, but it wasn’t until Monday that the authorities revealed the breadth of the disaster. Rescuers were able to save at least 69 people after the capsizing, officials said.

The ferry is thought to have been carrying 179 people, including crew members, according to Guyanese authorities, but the official manifest for the state-run ferry showed only 133 people aboard. That would mean that the vessel was carrying dozens more passengers than it should have been.

It was not immediately clear what caused the ferry to capsize. The captain and a crew member, both of whom were rescued, tested positive for cannabis after the capsizing, senior government officials said, which fueled speculation as to whether this may have contributed to the tragedy.

Some passengers were asleep at the time of the tragedy and told local media that they had just minutes to grasp life jackets or cling to floating objects like ice chests, which allowed them to survive.

The disaster has sparked outrage in Guyana, an oil-rich country of nearly 1 million people neighboring Brazil and Venezuela. Some in the English-speaking nation have questioned whether some passengers were allowed to buy tickets without having their names added to the manifest, enabling ferry employees to illicitly pocket the proceeds.

Guyana’s president, Irfaan Ali, said in a televised address on Monday night that the tragedy had “pierced the heart of our nation,” and he vowed that investigators would determine what caused the capsizing. Earlier on Monday, Mr. Ali traveled to sites near where the ferry capsized and met with survivors and relatives of those still missing.

“The families affected, and indeed the people of Guyana, deserve answers,” Mr. Ali said. “They deserve to know precisely what transpired, and whether any failures contributed to this heartbreaking event. We will pursue the truth with transparency, impartiality and accountability.”

Still, Mr. Ali said that finding survivors was the top priority and that government employees were working around the clock in one of Guyana’s largest maritime search operations. In addition to Guyana’s own rescue teams, the French government sent a 15-member squad with divers from its armed forces in neighboring French Guiana to assist in the search for survivors.