A year ago almost to the day, on 24 October 2012, a US drone strike killed a 68-year-old woman named Mamana Bibi. She was gathering vegetables in her family's large, mostly vacant fields in north Waziristan, Pakistan. We don't know whom the US intended to target, but it is hard to imagine that a policy that allows the killing of this grandmother, who was blown to pieces before the eyes of her young grandchildren, is anything but a catastrophic failure on the part of the US government.

The latest revelation from documents leaked by Edward Snowden, reported in the Washington Post, suggests the NSA cast a "surveillance blanket" over parts of northern Pakistan, feeding enormous amounts of data to the CIA's secret lethal drone program. Even if the NSA didn't pick up chatter after the killing of this grandmother, the US government claims that it conducts post-strike assessments of who is killed. It knew or should have known that something went wrong.

But like a hit-and-run driver, the US government never looked back. It never apologized or compensated Mamana Bibi's family. It never admitted what it did to the American public. Instead, top administration officials continued to tout the drone's precision capabilities. John Brennan, the architect of the drone program who now heads the CIA, has previously claimed that drone strikes caused absolutely no civilian casualties. He now admits they have, but says these deaths have been exceptionally rare.

Edward Snowden's revelations over the last few months make clear that government assurances like these cannot be trusted on faith alone. In the name of safeguarding national security, the government has withheld a whole universe of surveillance from the public. It has held back damning stories like the killing of Mamana Bibi. It may be holding back more horrific stories like this.

It is time to demand that the US government, and President Obama in particular, tell the whole truth about the US drone program: not just the claimed successes, but the human costs of its failures. Especially over the last two years, journalists and human rights groups have gathered credible documentation of civilian deaths from drone strikes, suggesting that Mamana Bibi's death is not an isolated incident. Amnesty International released a report Tuesday raising serious concerns about several recent drone strikes that appear to have killed civilians outside the bounds of the law.

The US government has never committed to investigating these cases. It has never even acknowledged responsibility for most of these strikes.

With an awesome power to gather information on the activities of citizens and non-citizens alike, the Obama administration has little excuse for failing to identify and investigate who it is killing in its drone strikes. Its continued silence about the deaths of people like Mamana Bibi suggests that it is either concealing such killings, or is completely indifferent to them.

In May 2013, the Department of Justice disclosed that four US citizens had been killed in drone strikes. It said that only one of them was an intended target; the other three, including a 16-year-old boy, were apparently unintended killings.

Yet, the Obama administration has never explained why these killings occurred. Nor has it ever told the public, or even most members of Congress, basic details, such as how many other people have been killed, the basis for their killing or their identities.

Since the Obama administration has not self-reported on its abuses, Congress must force its hand. It should investigate the drone program, including all credible allegations of civilian deaths, and call on administration officials to come clean.

None of this will bring Mamana Bibi back to her family. But justice for this grandmother – and all people unlawfully killed in the US drone program – begins with US officials admitting the truth they may already know.