KUALA LUMPUR, Malaysia — The lead coordinator of the search for Malaysia Airlines Flight 370 said Monday that after six days without detecting any further underwater signals that could be from the plane’s flight recorders, searchers planned to take the hunt to the dark depths of the Indian Ocean, deploying a submersible vehicle to scan the seabed for any signs of wreckage.

The absence of any more pings, taken together with the belief that the batteries on the flight recorders were at the end of their life span, has led the authorities to conclude that they are unlikely to detect any further signals and that they need to shift search tactics.

“It is time to go underwater,” the lead coordinator, Angus Houston, said at a news conference in Perth, Australia.

But striking a note of caution that has become a motif of his public appearances, Mr. Houston said there was no guarantee that searchers would find the wreck.