But now, Iraqi and U.S. officials say, the insurgent group has more than doubled in numbers from a year ago — from about 1,000 to 2,500 fighters. And it is carrying out an average of 140 attacks each week across Iraq, up from 75 attacks each week earlier this year, according to Pentagon data.

“AQI is coming back,” U.S. Sen. Lindsey Graham, a Republican from South Carolina, declared in an interview last month while visiting Baghdad.

The new growth of al-Qaida in Iraq, also known as the Islamic State of Iraq, is not entirely unexpected. Last November, the top U.S. military official in Iraq, Army Gen. Lloyd Austin, predicted “turbulence” ahead for Iraq’s security forces. But he doubted Iraq would return to the days of widespread fighting between Shiite militias and Sunni insurgents, including al-Qaida, that brought the Islamic country to the brink of civil war.

While there’s no sign of Iraq headed back toward sectarian warfare — mostly because Shiite militias are not retaliating to their deadly attacks — al-Qaida’s revival is terrifying to ordinary Iraqis.