Two people were arrested in connection with the fire, police spokesman Micky Rosenfeld said without providing further details about the suspects. "We're talking negligence at the moment," Rosenfeld said.
Israel Radio, citing unidentified police officials, said the blaze likely was started by a family that failed to extinguish a picnic fire.
The blaze, which broke out Thursday, tore through the hilly pine Carmel Forest that clings to the mountain ridge above the northern city of Haifa, Israel's third-largest city. It also lapped at the edges of small outlying communities.
The blaze has killed 41 people - most of them prison guards whose bus was engulfed by flames as they rushed to evacuate a prison. More than 17,000 people have been displaced.
Critics contrasted Israel's helplessness in the face of a wildfire with its reputation for swift and effective responses to disasters abroad.
"We are experts in some tragedies but it seems we were not really prepared for an event related to fire," said Hanan Goder, a spokesman for the Foreign Ministry, which is helping to coordinate the international response to the fire.
Israeli firefighters have complained for years of undersized crews, outdated equipment and minimal supplies. The country only has 1,400 firefighters, far below the worldwide average.
In the small artist community of Ein Hod, one woman desperately tried to protect her house from the encroaching flames with only a garden hose.
Another resident said he returned to his home after initially agreeing to evacuate.
"If we don't protect our house, nobody will," Gal Yarkoni told Israel's Channel 10.
International aid continued to flow into Israel in an effort to quell the fire, which continued Saturday to rage out of control for a third day.
U.S. planes laden with equipment were expected to land later Saturday, alongside aircraft sent from New York City's firefighting department, an Israeli military official said. French and Italian firefighting planes were also expected.
The aircraft are expected to join firefighters and planes that already have arrived from Bulgaria, Greece, Jordan, Turkey, Russia, Cyprus and Britain.
By Saturday, the fire had burnt some 15 square miles (40 square kilometers) of land - at least half of the Carmel national forest, Goder said.
While small by international standards, the loss of the verdant forest was keenly felt in Israel, where only 7 percent of the land is wooded.