Reporters from The Washington Post received Pulitzer Prizes in both the national reporting category, for their database profiling the 990 fatal police shootings in 2015, and in general nonfiction, for Joby Warrick’s "Black Flags: The Rise of ISIS.” (Erin Patrick O'Connor/The Washington Post)

After covering several high-profile incidents involving the killings of civilians by police officers in 2014, Washington Post staff writer Wesley Lowery was surprised to discover that there were no official statistics about such fatalities. So Lowery pitched an idea to his editors: The newspaper, he suggested, should collect the information itself and analyze it for patterns in law enforcement.

The Post soon marshaled an extraordinary team of reporters, editors, researchers, photographers and graphic artists to do just that. The result was a database containing the details of 990 fatal police shootings across the nation in 2015 and a series of articles describing trends in the data.

On Monday, The Post’s series was awarded the Pulitzer Prize in the national reporting category. The prizes, in their 100th year, are newspaper journalism’s highest honor.

The Pulitzer board also recognized “Black Flags: The Rise of ISIS,” a book by Post reporter Joby Warrick, in the general nonfiction category. The award was Warrick’s second Pulitzer; he previously won in 1996 for a series of articles about the environmental costs of North Carolina’s hog-farming industry, written with two colleagues at the News & Observer newspaper in Raleigh.

Spurred by Lowery’s proposal, the police-shootings project grew into one of the largest in the newsroom’s history, said Cameron Barr, The Post’s managing editor for news. It eventually involved some 70 journalists from the paper’s national, investigative, metro, video, photo and graphics departments.

The police-shootings database — painstakingly assembled by researchers Julie Tate and Jennifer Jenkins from official and unofficial sources — included more than a dozen details about each incident, including the age and race of the person killed, whether and how the person was armed, and the circumstances that led to the encounter with police. It soon yielded new insights into the use of deadly force by the nation’s police officers.

The data showed, for example, that about one-quarter of those fatally shot had a history of mental illness; that most of those killed were white men (although unarmed African Americans were at vastly higher risk of being shot after routine traffic stops than any other group); and that 55 officers involved in fatal shootings in 2015 had previously been involved in a deadly incident while on duty.

Another important finding: The vast majority (74 percent) of people shot and killed by police were armed with guns or were killed after attacking police officers or civilians or making direct threats. This finding countered the impression left by several high-profile fatalities that police routinely use excessive force. Staff writer Kimberly Kindy reported many of the major pieces.

The Post has continued to update its database during 2016 and is reporting on other patterns.

Because police are not required to report shootings of civilians, some “basic facts” were missing from the national conversation about the topic, said Martin Baron, The Post’s executive editor. He said the newspaper’s journalists sought to fill in the “enormous information gap” with the database and on-the-ground reporting. “The Post delivered on a core journalistic mission — telling the public what it needs to know,” Baron said.

The newspaper’s reporting has helped spur federal efforts to collect similar information from the nation’s 18,000 police departments and has been used by police chiefs in their efforts to overhaul their use-of-force policies.

The series was the ninth Pulitzer-winning project in which Tate, an ace researcher, played a role. It also was the ninth in which investigations editor Jeff Leen was involved.

“Black Flags,” Warrick’s study of the Jordanian terrorist Abu Musab al-Zarqawi, who was killed in 2006, grew out of Warrick’s coverage of the Arab Spring uprisings for The Post in 2011. Although some analysts viewed the uprisings as hopeful, Warrick said it reminded him of the fall of the Berlin Wall and communism in the late 1980s and early 1990s, which he covered as a young reporter for United Press International.

In both cases, “there was a euphoria . . . but you could see the downsides,” especially in the case of the Arab Spring, as terrorist groups rose to fill the deepening power vacuum.

Warrick began researching his book in early 2013 as the Islamic State began to claim territory in Syria and Iraq. He saw Zarqawi’s story as a vehicle to explain the unfolding chaos and the region’s recent history.

The book received glowing praise upon its publication in September, and it is being considered for a film adaptation, Warrick said.

The Post’s Eli Saslow was a finalist in this year’s Pulitzer competition in the feature-writing category for three stories: a profile of a young woman wounded in a mass shooting in Oregon, an article about a teenage single father and a piece about a family living in rural Nebraska a decade after being displaced by Hurricane Katrina.

Saslow, whose story on Oregon shooting victim Cheyeanne Fitzgerald won the Dart Award for Excellence in Coverage of Trauma earlier this month, won a Pulitzer for explanatory reporting in 2014 for a series on the federal food-stamp program.

Among other winners:

●The Associated Press won the gold medal for public service for its investigation of abuses of workers who supply seafood to American supermarkets and restaurants, including the use of slaves.

●The Los Angeles Times, in the breaking-news category, won for its coverage of the mass shooting in San Bernardino, Calif.

●The Tampa Bay Times won two awards, one for local reporting that exposed a school board’s “culpability in turning some county schools into failure factories” and the other for investigative reporting in a joint project on mental hospitals with the Sarasota Herald-Tribune.

●Alissa J. Rubin of the New York Times was awarded the international reporting prize for “thoroughly reported and movingly written accounts giving voice to Afghan women who were forced to endure unspeakable cruelties.” New York Times photographers also shared a Pulitzer with the staff of Thomson Reuters for photos of migrant refugees.

●The Boston Globe won two awards: Jessica Rinaldi in the feature photography category for a “raw and revealing photographic story of a boy who strives to find his footing after abuse by those he trusted,” and Farah Stockman for commentary.

●Two nonprofit digital news organizations, ProPublica and the Marshall Project, teamed up to win in the explanatory-reporting category with exposés about “law enforcement’s enduring failures to investigate reports of rape properly and to comprehend the traumatic effects on its victims.”

●The New Yorker magazine was also a double winner, under new rules that allow magazines to enter. Emily Nussbaum won for criticism and Kathryn Schulz for feature writing.