JAKARTA, June 21 (Xinhua) -- Indonesia's North Sumatra provincial police have detained the captain of an allegedly overloaded boat for questioning, after the vessel capsized leaving at least four people dead and over 192 others missing, national police spokesman said here Thursday.

Captain Situa Sagala is among the 18 people rescued after Kapal Motor Sinar Bangun boat sank in waters of Toba Lake in the province on Monday, national police spokesman Senior Commissioner Yusri Yunus said.

"He is among the 18, so we detained him. But we have not questioned him yet, as he is still traumatized," the spokesman said.

Another police spokesman Inspector General Setyo Wasisto noted that the captain would be questioned over the number of passengers as it is not clear yet with regard to the precise number of people in the boat.

The police would also seek clarification whether the boat was operating illegally as it had no tickets for passengers, said Wasisto.

The boat capsized at 5:30 p.m. local time (1030 GMT) on Monday after leaving Simanindo port in Samosir district and was 1.61 km away from its destination of Tigaras Parapat of Simalungun district.