(Reuters) - Clarita Alia knew what was coming.

Philippine President Rodrigo Duterte addresses an event with Filipino community in Hong Kong, China April 12, 2018. REUTERS/Bobby Yip

I met her with Reuters colleague Manny Mogato in May 2016 in a slum in Davao City, just after its long-time mayor, Rodrigo Duterte - also known as “The Punisher” - was elected president of the Philippines. Four of Alia’s sons were killed in Davao in a brutal anti-drug campaign that Duterte had vowed to take nationwide.

“Blood will flow like a river,” Alia predicted.

She was right.

Within months of Duterte taking office in June 2016, police or unidentified gunmen had killed thousands of drug suspects. And so, joined by another colleague, Clare Baldwin, we began our in-depth reporting of “Duterte’s War” that would win Reuters the Pulitzer Prize for international reporting.

Read the investigation here: here

At first, we focused on telling the tragic stories of drug-war victims and challenging false claims by the Duterte administration. In 2017, we set ourselves a bigger and riskier goal: to expose the killing machine behind those deaths – the Philippine police - and name the killers themselves.

We did this by melding data journalism, multimedia and shoe-leather reporting. Clare and I accessed and analyzed a trove of official crime reports, security camera footage and crime scene photos. This allowed us to identify not just new patterns in the killings, but also the top killers.

We corroborated our findings with months of reporting in slums and hostile police stations, often working as a team to watch each other’s back. Clare was greeted at one station by homicide detectives who shouted and lifted their shirts to display their guns.

Human-rights groups blamed thousands of vigilante-style killings on the police or their associates. The police publicly denied this. But two senior officers - one of them, at times, trembling with nerves - told Clare and Manny that police had carried out most of these killings. That story also cited a secret report, leaked to Manny, that detailed how police received cash for executing suspects, planted evidence at crime scenes and disabled security cameras in neighborhoods where they planned to kill.

What gave our stories their potency - and what so enraged the Duterte administration - was our use of the Philippine police’s own data, mainly in the form of crime reports, to undermine and disprove official claims. As 2017 went on, the police made it harder to get information about their deadly operations, while the government started its own campaign to counteract what it said was “fake news” about drug-war killings.

Even so, we continued to pry data from the police by making calls, writing letters and visiting stations to inspect and record blotters and other original documents. This allowed us to identify a deadly police unit from Duterte’s hometown - the “Davao Boys.” Armed with this data, Clare and I retraced the unit’s lethal path through Metro Manila’s streets. When Clare asked one Davao Boy why he had been chosen for the unit, he smiled and replied: “Special kill skills.”