Senate Intelligence Chair Sen. Dianne Feinstein, D-Calif. is pursued by reporters on Capitol Hill in Washington. (Manuel Balce Ceneta/AP)

In late 2007, not long after the Senate Intelligence Committee learned that the CIA had destroyed videotapes showing detainees being waterboarded, the spy agency for the first time allowed congressional staffers to review sensitive cables about the detention program.

The committee chose two staffers — a former FBI analyst named Daniel Jones and Alissa Starzak, who once worked as a CIA lawyer — to read thousands of cables from the agency. Soon the pair were sorting through a blizzard of documents numbering in the millions.

Seven years later, the Senate committee on Tuesday finally released a declassified summary of a 6,000-plus-page report on the CIA’s rendition, detention and interrogation program that offers a harsh assessment of one of the agency’s most controversial chapters.

[Read: Senate Intelligence Committee’s full report on the CIA program]

Sen. Dianne Feinstein (D-Calif.), the committee chairwoman, has said the report — which was initially expected to take one year to complete — constitutes the most important work the oversight committee has ever done and one of the most significant conducted in Senate history.

The Senate Intelligence Committee’s report on the CIA’s interrogation program listed, for the first time, the names of the 119 detainees who went through the agency’s secret prison system.

While the report involved a handful of committee aides, according to congressional sources and others, it owes its existence largely to Jones and Starzak, two staffers who worked quietly on the project while plunging into an environment plagued by mistrust between the Senate and the CIA.

[View timeline: The CIA’s use of harsh interrogation]

The effort, by all accounts, snowballed into a project that consumed them for years and was complicated by political obstacles.

There were “corrosive levels of draconian secrecy,” Andy Johnson, the committee’s former staff director, said at an event earlier this year in Washington. “The fog of secrecy made a mockery of oversight.”

Jones, 39, and Starzak, 41, have not spoken publicly about the study. Starzak declined to comment. The Intelligence Committee would not make Jones available for comment. Both are listed in the study’s acknowledgments.

The report remains the subject of deep divisions among those who have read it. Sen. Saxby Chambliss (R-Ga.), the panel’s ranking Republican, has derided it as “an ideologically motivated, distorted recounting of events.”

Some CIA veterans who were involved in the program have bristled that they were not interviewed as part of the Senate study and similarly concluded before the release that it would be deeply flawed.

Still, there is little disputing the time and resources put into the report, which is estimated to have cost more than $40 million and required Jones and Starzak to sift through a mountain of evidence.

“They had the analytical skills,” said Johnson, who tapped the pair to take the lead on the project. “Both had a very good understanding of the intelligence community. They were not partisan.”

Neither Jones nor Starzak were longtime veterans of the intelligence community. Jones had landed a job as an analyst in the FBI’s international terrorism operations section in 2004 after earning advanced degrees at Harvard and Johns Hopkins universities. He was an alumnus of Teach for America who had once been named to People magazine’s list of 100 Most Eligible Bachelors.

Starzak was a graduate of the University of Chicago Law School who had joined the litigation division in the office of the CIA’s general counsel after the Sept. 11, 2001, terrorist attacks. Her boss at the time, John Rizzo, said she handled significant cases but was not involved in the interrogation program. She left the agency on good terms, he said.

Both Jones and Starzak landed on the Senate Intelligence Committee staff in 2007. Jones was hired to conduct oversight on the FBI, and Starzak worked in the committee’s general counsel’s office.

Soon they were assigned to examine the cables that then-CIA Director Michael V. Hayden had made available after the destruction of the tapes.

Much of their work was conducted in the “cave,” a secret location in Northern Virginia where the CIA had set up a working space and computers to review agency documents, according to current and former Senate staffers.

Jones was known for sending e-mails between midnight and 2 a.m. and consuming an inordinate amount of coffee. He wrote thousands of pages in the report.

“He was a very careful examiner of a mass of evidence,” said one former GOP colleague, who like others spoke on the condition of anonymity because of the secrecy under which the investigation was carried out. “That’s the key. It’s an enormous amount of evidence out there, and he’s a person who is attentive to the facts.”

Jones and Starzak, along with Mike Davidson, the committee’s general counsel, spent nights and weekends on the project, cataloguing the documents and building a chronology of the CIA’s program. Feinstein ordered a full-blown review of the program in 2009. Other committee staffers playing central roles included Evan Gottesman, Chad Tanner and David Grannis.

In 2010, Jones and his colleagues came across a key CIA document — an internal agency inventory of records that seemed to corroborate the Senate panel’s findings. Created at the direction of then-CIA Director Leon E. Panetta, the inventory was not meant to be shared with the Senate.

Committee staffers printed and removed the document, taking it to the Hart Office Building on Capitol Hill, where they placed it in a safe.

When the CIA learned that the documents had been removed, the spy agency accused committee staffers of hacking into their computers and made a criminal referral to the Justice Department. The CIA’s inspector general later said in a statement that the allegations were “not supported” and the referral was dropped.

The threat of an internal inquiry, aides said, rattled Jones and other staffers involved in the Senate study. Even after the inquiry was shelved, the CIA and Feinstein continued to tussle over redactions to the report.

In 2011, a year before the completion of the Senate report, Starzak left the committee, taking a job with the Pentagon in the general counsel’s office. Jones is still on the committee, but after nearly eight years, friends say he has grown more cynical about Washington.

“Dan’s just shouldered a lot,” said the former GOP aide.

Julie Tate contributed to this report.