Robert O’Harrow Jr., a longtime investigative journalist at The Washington Post whose wide-ranging projects included probes into fiscal fraud, government contracts and internet privacy as one of the first reporters to explore emerging digital surveillance, died Dec. 4 at his home in Arlington, Virginia. He was 64.

The cause was cancer, said his wife, Amy Spector.

During more than two decades on investigative teams at The Post, Mr. O’Harrow was part of some of the most ambitious undertakings — hunkering down in his basement study (dubbed his “war room”) to analyze reports and datasets while also working the phones and meeting sources, using skills he honed earlier as a beat reporter.

“He always came back with something interesting and impactful,” said The Post’s former investigations editor Jeff Leen, who oversaw Mr. O’Harrow’s work from 1999 to 2023. “He had a gift.”

In the late 1990s, leading The Post’s first privacy beat, Mr. O’Harrow explored how personal and financial information was becoming an increasingly coveted commodity in cyberspace and leaving users exposed to hacking or losing control of their private information. His reporting in 1998 on the sharing of confidential prescription records prompted the drugstore chain CVS and the supermarket Giant to buy full-page ads in The Post to apologize to customers.

For the Jan. 1, 2000, edition of The Post, he wrote a prescient essay on how social interactions and personal information will one day flow through cyberspace: “Now that we’ve lived in the new millennium for a whole day, it seems sensible to begin worrying what great bugaboos it will bring. My candidate: the loss of privacy.”

Five years later, with the publication of his first book, “No Place to Hide: Behind the Scenes of Our Emerging Surveillance Society” (2005), many reviewers praised Mr. O’Harrow’s coolheaded clarity in explaining how modern life — from mobile phones to e-commerce — have become round-the-clock tracking networks. (In one personal decision to avoid digital breadcrumbs, Mr. O’Harrow rarely used credit cards.)

New York Times columnist William Safire described the powerful reporting in Mr. O’Harrow’s book as an alarm worth heeding, which “might just do for privacy protection what Rachel Carson’s ‘Silent Spring’ did for environmental protection nearly a half-century ago.”

In 2008, Mr. O’Harrow and colleague Brady Dennis brought readers deep into one of the major cautionary tales of the global financial crisis: how the insurance giant AIG manipulated derivatives, known as credit default swaps, and made other highly risky moves to create a financial time bomb that nearly destroyed the company.

Both projects were Pulitzer finalists — in 2000 for beat reporting on gaps in digital privacy; and in 2009 for Mr. O’Harrow and Dennis for explanatory reporting on AIG. Mr. O’Harrow also was part of The Post team awarded a Pulitzer in 2018 for investigative reporting into Roy Moore, a former Alabama high court judge whose bid for the U.S. Senate unraveled amid allegations of harassment of teenage girls and his attempts to discredit journalists covering the claims.

“Robert could see patterns that others couldn’t see, he could connect the dots that others couldn’t connect,” said former Post journalist Scott Higham, who worked with Mr. O’Harrow from 2005 to 2007 on a series of investigations into abuses within government contracting.

At the beginning of their reporting, Higham and Mr. O’Harrow attended a 10-day “contracting school” for civil servants and others involved in the process of writing and approving government contracts. The classes paid off, said Higham. He and Mr. O’Harrow were able to quickly build trust with sources who eventually leaked insider details. “We were able to speak their language,” Higham recalled.

One night, a source told Higham and Mr. O’Harrow to wait at a bus stop near The Post’s offices in downtown Washington. The source got off a bus and handed Mr. O’Harrow a shopping bag full of Pentagon contracting documents, Higham said.

The papers became part of a multiyear investigation with findings that included more than $100 million in cost overruns and waste in spending on security in the wake of the Sept. 11, 2001, terrorist attacks. Their work received an Investigative Reporters & Editors Award in 2005.

Another probe by Higham and Mr. O’Harrow led to the resignation of General Services Administration chief Lurita Alexis Doan in 2008. Their stories disclosed mismanagement such as Doan approving a $20,000, no-bid deal for a business run by a friend.

In 2010, Mr. O’Harrow wrote a series of stories showing how White business executives outside Alaska profited from government contracts intended to benefit Indigenous communities in Alaska. Then a year-long project in 2012, “Zero Day,” examined growing cybersecurity threats.

He joined Post colleagues Michael Sallah and Steven Rich in the 2014 project “Stop and Seize,” investigating how enforcement officers used federal asset forfeiture laws to seize billions in assets from motorists who were not charged with crimes. Attorney General Eric Holder later issued orders seeking to curb the abuses.

Speaking in 2016 at a cybersecurity summit hosted by The Post, Mr. O’Harrow offered a glimpse into how he tried to make sense of the possible perils and find targets for his investigative projects. He said he envisioned the digital world as full of “black holes” that lacked adequate privacy protections.

“When all of us around the world rely profoundly on cyberspace … for our social interactions and our national security, and our power grids and our credit grids,” he said, “how do we fill in those black holes?”

Began as copy aide

Robert Emmett O’Harrow Jr. was born in Indianapolis on July 16, 1960. His father worked in data communications sales and marketing, and his mother was a hotel sales manager.

He graduated from Virginia Tech in 1982 with a bachelor’s degree in economics and history — and had his introduction to journalism as a copy aide at The Post. He worked as a reporter at the Times Herald-Record in Middletown, N.Y., before joining The Record in Bergen County, New Jersey, in 1989.

He was back at The Post in July 1990 as a Metro reporter in Virginia’s Loudoun County. Following a tip, he helped uncover a private security group formed to provide armored vehicles to law enforcement agencies. The disclosures led to the arrest and conviction in 1995 of the group’s founder, the scion of a wealthy local family, for impersonating a federal officer.

When Mr. O’Harrow approached investigations editor Leen about joining his team, Leen was initially cautious. Leen said he wondered if Mr. O’Harrow was more attuned to beat reporting and whether he may chafe at spending months on an investigation without having a byline in print. “I guess I was proven wrong,” quipped Leen.

Mr. O’Harrow’s other books include “Zero Day: The Threat in Cyberspace” (2014), and “The Quartermaster: Montgomery C. Meigs” (2016), a biography of the logistics and supplies chief for the Union Army during the Civil War. Mr. O’Harrow also contributed to “Trump Revealed: An American Journey of Ambition, Ego, Money, and Power” (2016), written by Post colleagues Michael Kranish and Marc Fisher.

Among Mr. O’Harrow’s awards were the Carnegie Mellon Cybersecurity Reporting Award in 2003, and the Sigma Delta Chi Award in 2013, for the “Zero Day” series.

In 2017, he joined The Post’s Rapid Response investigative team, where he collaborated on stories that included alleged financial abuses and sexual improprieties by the Roman Catholic bishop of West Virginia. Mr. O’Harrow retired from The Post in early 2022 but remained as a contributing writer.

During an appearance by Mr. O’Harrow on “The Daily Show” in 2006 to discuss government and corporate cybersurveillance, host Jon Stewart turned to the audience in mock panic: “We’re going off the grid … let’s do it!”

Mr. O’Harrow deadpanned: “Uh, yeah. Good luck.”

Mr. O’Harrow’s marriage to Christina Campanella ended in divorce. He married Amy Spector in 2012. Other survivors include two children, Ana and Cormac, from his first marriage; two stepdaughters, Michela Monday and Sidney Monday; his father Robert O’Harrow Sr.; sister Lisa O’Harrow-Ray; and brother Kevin O’Harrow.

On Twitter (now X), Mr. O’Harrow made clear he was always looking for his next big project. “Ideas?” he wrote on his bio blurb. He gave a number with the encrypted mobile service Signal for any tips.