JAKARTA, Indonesia — The head of the Indonesian agency searching for a missing AirAsia jet carrying 162 people said Monday that he believed the aircraft was “at the bottom of the sea” and warned that the country lacked adequate equipment to conduct an underwater search.

“The capability of our equipment is not optimum,” Bambang Soelistyo, the head of Indonesia’s National Search and Rescue Agency, said at a news conference.

Malaysia, Singapore and Australia joined the search, an effort that evoked a distressingly familiar mix of grief and mystery nine months after a Malaysia Airlines jetliner disappeared over the Indian Ocean.