Marc Garlasco headed the United Nations Protection of Civilians office in Afghanistan in 2011 and was the U.N. senior military adviser for the Human Rights Council’s Independent Commission of Inquiry on Libya where he led the investigation into NATO’s actions during the war in Libya.

As the international community assesses the situation in Syria, it’s important to keep in mind what might be expected of NATO.

I investigated the organization’s actions in Libya last year while at the same time working with the United Nations’ civilian protection office in Afghanistan. The difference in how NATO interacted with me in each place was striking. I had a collegial, open relationship with officials in Afghanistan — and an adversarial and frosty one with those in Libya.

In August 2011 laser-guided bombs dropped by NATO forces reduced three homes in Majer, Libya, to rubble. The strike killed 34, the largest loss of civilian lives from a NATO attack; 38 others were wounded. As the head of the U.N. investigating team for all NATO activity in Libya, I sifted through the debris and makeshift memorials in the small rural village near Misrata while interviewing survivors, trying to piece together what had happened. It didn’t make sense — NATO hit the homes and returned for a follow-up strike, killing the rescuers who were frantically digging for survivors minutes after the first bombs struck. A pilot using laser-guided bombs would have been able to see the rescuers at work.

I wanted to talk with NATO officials to understand the incident and help initiate the process for lessons learned from the civilian casualties, but NATO refused to meet. NATO officials have not considered civilian casualties in their May review of the war and have refused to conduct an on-the-ground assessment of the civilian impacts in Libya. These issues aren’t going away. How can NATO uphold its mandate to protect civilians while denying civilian harm?

I had a vastly different experience with NATO in 2008, when I investigated civilian deaths from airstrikes in Afghanistan for Human Rights Watch. At that time, civilians were being killed at a dizzying pace; the United Nations documented 359 civilian deaths from airstrikes in that year. After our report was released, NATO acknowledged problems and implemented changes: The number of documented civilian deaths stemming from airstrikes in 2011 was 187. When I led the U.N. office on civilian protection in Kabul last year, I saw firsthand how seriously alliance forces take civilian casualties.

Last July, for example, our Gardez office alerted me that a night raid in Khost had killed a number of civilians. Alliance troops initially told me they had hit a Haqqani network commander and killed him, as well as a woman who had been targeting Western forces and at least four other “militants.” The U.N. team on the ground said it had proof this was wrong and provided evidence that compelled the launch of a joint incident-assessment team. This ad hoc assembly of military professionals partnered with Afghan investigators on a thorough investigation. The U.N. liaison officer to the International Security Assistance Force (ISAF) called me later to confirm they had unfortunately killed civilians and thanked us for repudiating their initial claim. A colonel with the ISAF subsequently participated in a jirga in the village to apologize and offered amends for the dead. It is hard to believe NATO has not applied in Libya simple lessons gleaned from a decade in Afghanistan: investigate civilian casualties, acknowledge and make amends.

Although civilian casualties were reported in only nine targets out of more than 7,700 dropped bombs in Libya in 2011, NATO refused to discuss those incidents or provide gun-camera footage and questioned why the United Nations was investigating. Alliance officials eventually declassified information that was provided to my team, but they gave terse answers to our queries and claimed that all targets were legitimate and that no civilian casualties could be confirmed.

The United Nations provided Global Positioning System coordinates for each strike, dates and times of attacks, specifics on the bomb fragments found at each site, details about the civilians killed and the lack of any military signatures such as weapons or communications equipment that would have proved these were legitimate military targets. NATO responded that we had gone to the sites too late to find any proof of military targets. But if I could find the remnants of NATO bombs months after some strikes, surely there would be remnants of the intended targets. Instead I found only the remains of homes.

The action in Libya was launched for principled ends, succeeded in protecting Libyans from Moammar Gaddafi’s atrocities and did so while causing very few civilian casualties. Still, NATO is wrong to say it does not have to investigate civilian casualties. For one thing, after NATO airpower turned the war in the favor of the rebels in Libya, it should not be difficult to gain access to nine sites. But alliance officials should want to improve targeting to prevent civilian deaths. If NATO intelligence was wrong in these instances, what produced the error? And with some floating the possibility of NATO intervention in Syria, shouldn’t the alliance recognize its opportunity to verify the legitimacy of targets and better ensure that civilians are spared?

Instead of dealing with the United Nations investigation openly and admitting mistakes, NATO’s secrecy has fed Chinese and Russian conspiracy theories of massive civilian casualties. NATO Secretary General Anders Fogh Rasmussen even said in March that the operation in Libya caused no confirmed civilian casualties. I suggest he tell that to the families in Majer.