Rescuers were searching Thursday for 30 people who were missing after a ferry sank and four people died the previous night near Indonesia’s resort island of Bali.

As of Thursday afternoon, 31 people had been rescued from the ferry’s 53 passengers and 12 crew members, the National Search and Rescue Agency said in a statement.

The KMP Tunu Pratama Jaya sank almost half an hour after leaving Ketapang port in the East Java town of Banyuwangi late Wednesday for a 50-kilometer (30-mile) trip to Bali’s Gilimanuk port, authorities said.

A helicopter and nine boats including two tug boats and two inflatable boats searched for survivors with assistance from fishermen and people onshore.

Strong waves up to 2 meters (6.5 feet) high and darkness hampered emergency responders overnight, but an official said improved weather and sea conditions Thursday morning were assisting the search effort.

“For today’s search we are focusing on searching on the water, as the initial victims were found in the water between the location of the accident toward Gilimanuk port,” Surabaya Search and Rescue head Nanang Sigit said in a statement.

An officer at the port witnessed the sinking before rescuers could be alerted.

“The ferry could not be contacted via radio from the beginning. Then it could be contacted by other ships from the same company. But the ship was already in a tilting condition,” Sigit said.

Many of those rescued were unconscious after drifting in choppy waters for hours, Banyuwangi Police Chief Rama Samtama Putra said.

Indonesian authorities are investigating the cause of the accident. Survivors told rescuers there appeared to be a leak in the engine room of the ferry, which was carrying 22 vehicles including 14 trucks.

Some family members arrived at the port in a panic or weeping as they sought information about their loved ones.

Survivors were taken to nearby medical facilities including Jembrana Regional Hospital in Bali.

“When the ferry started to tilt, I initially intended to jump into the sea, but the ship quickly sank, so I did not jump any more but sank with the water entering the ship, maybe about 7 meters (23 feet) deep, so I immediately climbed up to the top,” said Supardi, 64, a survivor at the hospital.

He and three other people grouped together in the water and used life jackets to say afloat, he said.

Ferry tragedies are common in Indonesia, an archipelago of more than 17,000 islands, where ferries are often used as transport and safety regulations can lapse. (CNN)