CAIRO — In just the last two weeks, Islamist militants have detonated a car bomb at the gates of the capital’s security headquarters, gunned down a senior Interior Ministry official in broad daylight and shot down a military helicopter over Sinai with a portable surface-to-air missile.

But perhaps most alarming to officials in Cairo and Washington are the signs that the swift increase in the scale and effectiveness of the attacks may come from a new influx of fighters: Egyptians returning from jihad abroad to join a campaign of terrorism against the military-backed government.

“Egypt is again an open front for jihad,” said Brian Fishman, a researcher in counterterrorism at the New America Foundation in Washington. “The world is being turned on its head, and, for the United States, the ability to rely on Egypt as a stabilizing force in the region — rather than a source of problems — is really being challenged.”

The birthplace of political Islam, Egypt has sent fighters to battle zones from Kandahar to the Caucasus for decades, and in the late 1990s its security forces crushed an Islamist insurgency at home. But until last summer, when the military ousted President Mohamed Morsi and began a bloody crackdown on his allies in the Muslim Brotherhood, Egypt remained largely insulated from the Islamist violence that flared up around it.