The disturbing phone call came after Baraa Shiban investigated a drone strike on a wedding party that killed 12 people in central Yemen in December. A clear message was delivered to the human rights researcher over the phone after a major news network reported the story based on his research.

"The caller refused to identify himself and threatened my life if I continued my investigation of the strike," Shiban told Al Jazeera, noting he conducted similar studies of US drone operations in the past, but had never before received death threats.

Shiban works for the UK-based human rights group Reprieve and interviewed survivors two days after the attack. His investigation ascertained that 12 people were killed after four missiles were fired at the convoy. There were also 14 victims with severe wounds; some lost limbs, others their eyes.

Along with the eyewitness testimony, Shiban gained access to video and still images of the alleged victims of the drone strike. Photos of the aftermath of drone attacks - whether in the tribal regions of Pakistan, or in the deserts of Yemen - are rarely captured. Most occur in obscure regions with hostile terrain, making access difficult for journalists and activists.

I was in the front car and I heard a huge explosion. I went out to see what happened and suddenly another two missiles hit the place. Everyone in the car behind us got killed. - Mohammed Abdullah al-Taisi, drone victim

On December 12, 2013 , about 60 people were traveling in a convoy to attend the wedding near the city of Radda, in Yemen's central province of al-Bayda. At about 4:30pm, the drivers halted the vehicles when they heard an aircraft approach.

"I was in the front car and I heard a huge explosion," recalled victim Mohammed Abdullah al-Taisi. "I went out to see what happened and suddenly another two missiles hit the place. Everyone in the car behind us got killed."

Equipped with the evidence Shiban went to the media, and a day later he received the call threatening his life.

"Just because the people were in a convoy of trucks, they were assumed to be militants and the decision was made to target them," he said. "The people who died were shepherds and farmers. There was clearly a wedding party."

Fear and anger

Drones piloted by the CIA and the Pentagon have operated in Yemen since 2002, killing hundreds of people - mostly members of al-Qaeda in the Arabian Peninsula, but also dozens of civilians.

Peter Schaapveld is psychologist who traveled to Yemen to study the programme's effects. He told British members of parliament in March 2013 that the constant presence of drones in the skies was causing a "psychological emergency" in the country.

"What I saw in Yemen was deeply disturbing," Schaapveld said . "Entire communities - including young children who are the next generation of Yemenis - are being traumatised and re-traumatised by drones. Not only is this having truly awful immediate effects, but the psychological damage done will outlast any counter programme and surely outweigh any possible benefits."

Reports of the missile strike, on a seemingly innocent wedding party, have infuriated nearly every sphere of Yemeni society, including many of the country's top politicians.

Family members of drone strike victim Aref al-Shafee [Abubakr al-Shamahi/Reprieve]

"The fact that the Yemeni parliament has just passed a resolution banning drones in Yemeni airspace, and that the National Dialogue has criminalised the use of drones for extrajudicial killing, demonstrates that a national consensus has been reached that these brutal and unlawful attacks are unacceptable," Shiban said.

Reprieve said the US government is now investigating the strike in Radda following Shiban's work. The human rights group said the Defense Department was targeting Shawqi Ali Ahmed al-Badani , whom the White House accused of organising a bomb plot that led to 19 US embassies being closed last year.

'US values and policy'

Caitlan Hayden, a spokeswoman for the US National Security Council, noted that Yemen's government had stated the targets of the operation were "dangerous" senior al-Qaeda figures. She said she couldn't comment on this specific attack.

"We take extraordinary care to make sure that our counterterrorism actions are in accordance with all applicable domestic and international law and that they are consistent with US values and policy ... And when we believe that civilians may have been killed, we investigate thoroughly," Hayden told Al Jazeera .

But one survivor of the December drone attack, Salam al-Taisi, insisted no one from the wedding party was involved in terrorism. "None of the victims had anything to do with al-Qaeda or any other group. They were all from the area and all were poor villagers," he said.

The deaths in Baydah have more resonance considering President Barrack Obama's announcement upholding the "highest standard" when conducting operations using unmanned aerial vehicles.

"Before any strike is taken, there must be near-certainty that no civilians will be killed or injured," Obama said in a speech at the National Defense University on May 23, 2013.

Yemen's security forces have also scrutinised Shiban's work on the US drone programme. But it's not just the Yemeni police that have shown interest in him.

Baraa Shiban from human rights group Reprieve [Al Jazeera]

On September 23 last year, he arrived in the United Kingdom with the intention of speaking at a conference at Chatham House . But at Gatwick Airport he was stopped by police and questioned under Schedule 7 of the British government's Terrorism Act 2000.

"I was asked about my investigation of the covert US drone attacks in Yemen. When I asked why the question was relevant, I was threatened with further detention," Shiban said.

Drone-reporting dangers

Apparent attempts to suppress any kind of criticism of US covert operations are not new.

In Pakistan, an anti-drone campaigner set to testify before European parliaments has gone missing in the city of Rawalpindi. Kareem Khan , whose brother and teenage son were killed in a drone attack in December 2009, was picked up at his home by security forces in the early hours of February 5, his lawyer said. He hasn't been heard from since.

Shiban said he is also well aware that the path he's on now could lead to the same fate of Yemen-based journalist Abdulelah Haider Shaye .

On December 17, 2009, the Yemeni military announced it had successfully destroyed an al-Qaeda camp in al-Majala in Abyan province. But after travelling to the town, Shaye discovered it wasn't at all an operation carried out by his government, but in fact a US cruise missile strike. And he discovered the people who died weren't al-Qaeda fighters but innocent civilians. Among the 41 people killed, more than two-thirds were women and children.

Shaye was arrested on August 6, 2010 by Yemeni security forces and charged that October with aiding al-Qaeda by recruiting new operatives for the group. By January 2011, he was convicted and sentenced to five years in prison.

International human rights groups condemned his trial as a sham , which couldn't provide any credible evidence of his alleged al-Qaeda associations. Shaye was being punished for exposing a US covert operation that resulted in a massacre.

After being incarcerated for nearly three years, Shaye was pardoned in July 2013 but one of the conditions of his release is he must not leave the Yemeni capital, Sanaa, for two years.

Asked about Shaye's case, and the threats he's received to his own life, Shiban said he's determined to carry on highlighting the impact of drone strikes.

"This is an issue of vital importance to Yemen's future, and I and other human rights activists will continue to defend the basic rights and democratic wishes of the Yemeni people," he said.