JERUSALEM  A 14-year-old boy was arrested Monday as the prime suspect in the largest fire in Israel’s history, a four-day inferno that left 42 people dead, devoured 10,000 acres of forest and forced Israel to request international assistance.

The boy, from the Carmel area, where the fire began, admitted under questioning that he had been smoking a tobacco water pipe, or narghile, and had thrown away a hot coal that set off the fire, said Micky Rosenfeld, a police spokesman. He fled the scene, and without sounding any alert, went back to school, Mr. Rosenfeld said.

Two other boys from the same area, 14 and 16, who the police said were suspected of starting the wildfire through negligence, were released to house arrest on Monday. The two, who are brothers, were not identified because they are minors. Relatives and their lawyer have denied that the boys were involved in starting the blaze, and their connection to the third boy was unclear.

The fire, which broke out in the forested hills near Haifa, in northern Israel, on Thursday, was mostly extinguished by Sunday evening, with officials crediting assistance from an international fleet of more than 30 firefighting aircraft and the so-called Supertanker, the world’s largest fire-extinguishing plane.