A U.S. Predator drone flies over Kandahar Air Field, southern Afghanistan on a moonlit night.Kirsty Wigglesworth / AP

The United States drone war in Pakistan reached another grim milestone on Thursday with the announcement that drone strikes had inadvertently killed two hostages of al-Qaeda. The attack, carried out in January, struck a rural Pakistani compound concealing two Western aid workers—Warren Weinstein, an American, and Giovanni Lo Porto, an Italian. The White House also announced two American-born al-Qaeda militants, including Adam Gadahn, a high-profile spokesman for the terror group, had been killed by drones the same month, Gadahn in a separate incident.

In a statement delivered to White House reporters on Thursday morning, President Obama expressed regret for the attack and issued an apology to the families of Weinstein and Lo Porto.

"It is a cruel and bitter truth that in the fog of war generally and our fight against terrorists specifically, mistakes—sometimes deadly mistakes—can occur," he said.

The deaths of Weinstein and Lo Porto are likely to renew controversy surrounding drone warfare, which has served as a central plank of Obama's counterterrorism strategy since his 2009 inauguration. Small, pilotless aircraft controlled remotely, drones are reputed by their advocates to avoid the collateral damage associated with traditional airstrikes. Critics of drone warfare, however, allege that the strikes have killed far more civilians than targeted militants—a claim disputed by the Obama administration.

"The only people that we fire a drone at are confirmed terrorist targets at the highest level after a great deal of vetting that takes a long period of time," Secretary of State John Kerry said in 2013. "We don’t just fire a drone at somebody and think they’re a terrorist," he added.

But even at their most precise, drone strikes can only be as accurate as the intelligence they're based on. According to the Wall Street Journal, the CIA surveilled the Pakistani compound that held the two hostages for weeks prior to the January strike, monitoring every individual who entered or left the premises. But U.S. intelligence did not detect Weinstein and Lo Porto. In February, the U.S. received intelligence that the two men had died, and an investigation revealed that the drone strike—rather than an unrelated offensive by the Pakistani military—was responsible.

The 72-year-old Weinstein, a lifelong development worker, was kidnapped from his home in Lahore, Pakistan, in 2011 and taken hostage. Two years later, he appeared in a video released by al-Qaeda's media arm and pled for the U.S. to negotiate for his release. Washington, citing official policy against negotiating with al-Qaeda, refused. In a statement released on Thursday, Weinstein's wife expressed frustration with the U.S. hostage policy.

"We hope that my husband’s death and the others who have faced similar tragedies in recent months will finally prompt the U.S. government to take its responsibilities seriously and establish a coordinated and consistent approach to supporting hostages and their families," she said.

In his remarks on Thursday, President Obama expressed his horror at Weinstein and Lo Porto's death and appeared open to reconsidering his administration's policy.

"We will identify the lessons that can be learned from this tragedy, and any changes that should be made," he said. "We will do our utmost to ensure it is not repeated."

Throughout his presidency, Obama has defended drone strikes as legal, effective, and necessary. But whether this position remains tenable depends, as in all wars, on traditional intelligence gathering.

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