“All the pilots there operate under the assumption that they can be targeted,” Mr. Yaari said. “We have known for a long time that those guys in the Sinai have a lot of manpads from Libya.”

In Egypt, news of the attack was almost overshadowed by other political violence. The military-backed government held a celebration in Tahrir Square on Saturday to commemorate the third anniversary of the revolt that toppled President Hosni Mubarak, and at least 49 people were killed and 250 injured in clashes with security forces at protests against the military takeover, health officials said on Sunday. An earlier report had put the death toll at 29. Then, on Sunday night, a spokesman for the medical examiner said in a television interview that at least 62 people had been killed in greater Cairo alone.

Security officials reported more than 1,000 arrests.

On Sunday morning, masked men in an SUV attacked a bus carrying soldiers in Sinai, killing at least three and injuring at least 11, a military spokesman said. On Friday, at least six people were killed in four bomb blasts around Cairo, an art museum was badly damaged, and eight more people died in clashes with security forces.

The violence did not appear to slow the government’s brisk transition plan. Adly Mansour, a senior judge and Egypt’s interim president, condemned the “heinous terrorist attacks” in a seven-minute televised address, but he also accelerated the political calendar. Instead of holding parliamentary elections first as promised, Mr. Mansour decreed, the government will hold presidential elections first, sometime between Feb. 17 and April 18.

He offered little explanation, but the shift was widely expected. It will make it easier for Gen. Abdul-Fattah el-Sisi, who appointed Mr. Mansour as interim president, to run for president and consolidate his power before any rival parties or players can find footholds in Parliament.

In an effort to create an atmosphere of celebration for the third anniversary of the revolt — an event that doubled as a rally for General Sisi — the government declared the downing of the helicopter an accident until further investigation.

On Sunday, General Sisi attended a funeral for the five soldiers killed, and a statement from a military spokesman did not mention the cause of the episode, saying only that “one of the Air Force’s helicopters crashed while carrying out the raids in order to assist the military operations led by the Armed Forces against the Islamist extremist elements in North Sinai.”