Russian divers on the Volga River are probing a sunken tour boat where they expect to find the remains of scores of children entombed in a play area of the stricken vessel, which went down Sunday with 208 people on board.

The grim search was underway late Monday, as the confirmed death toll rose to 64.

The children are among about 100 people still missing after the tour boat abruptly sank in bad weather 750 kilometers east of Moscow in a huge river reservoir near the city of Kazan (in the republic of Tatarstan).

Russian television said the search was shifting to a recreation area aboard the stricken vessel where as many as 60 children were thought to be playing when disaster struck.

Russian President Dmitry Medvedev declared a national day of mourning for Tuesday, as he convened a special meeting of senior ministers and announced a formal probe into the sinking. He referred to the doomed boat as “decrepit” and one of a “number of old rust tubs we have sailing.”

Survivors say the overloaded boat started listing to its side within moments of setting sail in heavy rains and high winds. They say it sank in about three minutes, giving the crew little time to take action.