CIA Director John Brennan, left, and National Security Agency Director Michael Rogers listen to remarks by President Obama on Friday at the headquarters of the Office of the Director of National Intelligence. (Yuri Gripas/Reuters)

The vehicles were separated only by several car lengths when the missile struck.

The main target of the CIA drone strike in April last year was a Toyota carrying 11 armed men suspected of being part of an al-Qaeda plot to attack a military outpost in central Yemen. But the shrapnel also sprayed a truck traveling a few dozen yards ahead, killing or wounding nine laborers on an early morning commute.

The Yemeni government soon acknowledged that civilians had been killed in an operation it did not attribute to the United States, and human rights researchers were able to reconstruct the incident from witness accounts. But the U.S. government has yet to admit that the strike ever occurred.

That policy of silence is under renewed pressure after President Obama’s extraordinary admission Thursday that the United States had accidentally killed two Western hostages, including a U.S. citizen, in a January counterterrorism strike on a gathering of suspected al-Qaeda militants in Pakistan.

Speaking from the White House on April 23, President Obama said government officials had worked "tirelessly" to return Warren Weinstein to the U.S. before he was accidentally killed by a U.S. operation while being held by al-Qaeda. (AP)

The revelation has revived questions about why the White House has been unwilling to provide similar information on dozens of other strikes over the past decade where there is abundant evidence that civilians were killed.

“These disclosures have to come every time an innocent life is lost through the drone campaign, and not just when it’s an American citizen,” said Jonathan Horowitz, a legal officer for the Open Society Justice Initiative, which issued a report this month detailing civilian casualties in a series of U.S. strikes in Yemen, including the April 2014 operation.

More broadly, Thursday’s disclosures have also complicated the administration’s ability to continue depicting the drone campaign as nearly impervious to error, dismissing independent groups’ casualty estimates as wildly overstated.

In a speech before members of the U.S. intelligence community on Friday, Obama vowed a thorough probe of the Jan. 15 strike that killed U.S. aid worker Warren Weinstein and Italian national Giovanni Lo Porto, both of whom had been held hostage by al-Qaeda for years.

“We’re going to identify the lessons that can be learned and any improvements and changes that need to be made,” Obama said in a speech marking the 10-year anniversary of the creation of the director of national intelligence position, which oversees U.S. spy agencies. “We all grieve when any innocent life is taken.”

But Obama also signaled that this week’s disclosures are not likely to lead to a further lifting of the secrecy surrounding the drone program. “A lot of our work still requires that we maintain some things as classified,” Obama said. “We can’t always talk about all the challenges.”

In his announcement about the hostage deaths, Obama expressed regret and said the men were killed in a counterterrorism operation. But he did not acknowledge that they died in a drone strike.

A U.S. government contractor kidnapped by al-Qaeda militants in Pakistan in 2011 called on the Obama administration to negotiate with his captors and says he feels “totally abandoned and forgotten.”

The U.S. government has never publicly disclosed its own count of the number of deaths attributable to drone operations outside the war zones of Iraq and Afghanistan over the past 14 years. But U.S. officials involved in the operations have often claimed that civilians account for only 1 or 2 percent of those killed.

That ratio is generally at odds with the estimates of independent organizations that have sought to track the toll of drone strikes through media reports and on-the-ground research.

The Bureau of Investigative Journalism, for example, has documented 415 strikes in Pakistan and Yemen since the Sept. 11, 2001, terrorist attacks. The organization’s most recent estimates put the total number killed between 2,449 and 3,949. Of those, between 423 and 962 are believed to have been civilians.

CIA veterans scoff at such figures.

“I can guarantee you that those numbers are significantly higher than reality,” Michael Morell, former deputy director of the agency, said in an interview. Morell also said that civilian casualty figures would likely be significantly higher if the United States were forced to rely on conventional aircraft or ground troops.

“The alternative is much worse in terms of collateral damage,” Morell said.

Even critics of the drone program acknowledge the remarkable accuracy and capabilities of Predator and Reaper aircraft, which are able to linger over targets far longer than piloted aircraft, allowing for greater patience and certainty before launching missiles.

Those attributes have helped the drone program achieve a special status on Capitol Hill, with staunch support from key lawmakers in both parties.

The Senate Intelligence Committee, for example, issued a blistering report earlier this year that accused the CIA of widespread abuses in its interrogations of al-Qaeda suspects after the Sept. 11 attacks. But Sen. Dianne Feinstein (D-Calif.), who presided over that probe, has been such an ardent supporter of the CIA drone program that she has fought proposals to transfer control of it to the Pentagon.

The agency has also sought to protect that turf. When a drone strike carried out by the U.S. military’s Joint Special Operations Command allegedly struck a wedding procession in Yemen in 2013, the CIA produced a harshly critical assessment that infuriated U.S. military leaders, current and former U.S. officials said.

The CIA’s assessment was at odds with the Pentagon’s efforts to defend the strike and contributed to the administration’s decision to suspend JSOC’s authority to carry out such strikes. That suspension was only recently lifted, according to officials who said that JSOC was responsible for strikes in Yemen over the past week.

The deaths of Weinstein and Lo Porto, however, have put the CIA on the defensive over an error egregious enough to require a public admission from Obama.

U.S. officials said that the CIA program has not been suspended and that the review ordered by Obama will be carried out by the agency’s own inspector general. Still, some said the agency is in a newly vulnerable position, facing scrutiny that could expand if significant problems are uncovered.

One U.S. official described the disclosures this week as “cracks and fissures” in a program whose flaws once seemed minuscule. “The president only comes out when there is an Italian and U.S. hostage killed,” the official said, speaking on the condition of anonymity citing the secrecy of the program. “How many other times could the president have come out and said the same thing about other individuals who happened not to be U.S. citizens?”

In the report it released this month, the Open Society Justice Initiative identified civilians killed in the April strike in Yemen and included accounts from survivors and witnesses. Among them was Hussein Ahmed Saleh Abu Bakr, described as a 24-year-old laborer.

“The distance between our car and their car was about 20 to 30 meters,” Abu Bakr said, referring to the trailing vehicle carrying armed militants. “While we were chatting about work and its problems, we suddenly heard two explosions.”

The blast killed four and wounded five more traveling in Abu Bakr’s vehicle, according to the Open Society Justice Initiative. “This study found no credible indication that any [of those killed in Abu Bakr’s vehicle] . . . were associated with any terrorist group,” the report said. The organization was unable to confirm the identities of those traveling in the targeted vehicle, but it said that “it appears that they were al-Qaeda members” and, according to Abu Bakr, “seemed to have been killed on the spot.”