Twenty-five years after the September 11 attacks, the CIA has released a series of presidential briefing documents showing what United States leaders had been told about Osama bin Laden and al-Qaeda in the years before the plane hijackings that killed nearly 3,000 people.

CIA Director John Ratcliffe called the release of the documents “an act of exceptional transparency”. This is the agency’s largest-ever release of presidential daily briefs, the classified intelligence summaries written for the president and his most senior officials.

The files cover the period from February 1998 to September 12, 2001 – the day after the deadly attacks on New York City and Washington, DC – and reveal how the CIA tracked bin Laden’s movements, his financing and his networks inside the US and abroad.

In one, a briefer warns that the group behind that year’s East Africa embassy bombings “may fly an explosive-packed airplane into a US city”.

Another notes that some in bin Laden’s network had received hijack training. Others describe his hunt for materials to build a radioactive “dirty bomb”.

The documents add little new information about how the attacks were planned, but they do offer a paper trail of accumulating warnings.

Here’s a breakdown of what is in them:

What has the CIA just released?

Seventy-one presidential daily briefs spanning the period from February 1998 to September 2001. These are the highly classified intelligence summaries prepared for the president and the government’s most senior officials.

The documents run to 101 pages, with the earliest, titled Terrorists seeking WMD capability, dated February 13, 1998.

The last, Situation Report: Bin Laden’s Tracks, is dated September 12, 2001, the day after the attacks.

In between are briefs on the Taliban’s refusal to give bin Laden up, his hunt for chemical, biological and radiological weapons, his financing, and repeated warnings that he intended to strike inside the US.

Many pages are redacted to protect sources.

What did US intelligence know about al-Qaeda before 9/11?

A great deal, and from quite early on.

The briefs show that analysts were tracking bin Laden’s movements, the support system around him, his network inside the US and abroad, and how his operations were financed.

Some of the warnings laid out in the presidential briefs were strikingly specific. In September 1998, three years before the attacks, a briefer wrote that bin Laden’s “preferred option is to strike the US on its own soil in Washington”.

Analysts also noted his interest in unconventional weapons: Chemical, biological, and radiological.

By June 1999, the briefs included reports that bin Laden’s operatives had experimented with explosives to “increase their energy output”, part of a pursuit of so-called “dirty bombs”.

The obstacles were understood, too.

An April 1998 brief found there was no evidence the Taliban intended to hand bin Laden over, describing him as their “guest” and noting that “gratitude for his assistance during the Soviet occupation” of Afghanistan between December 1979 and February 1989 left them “reluctant to alienate him by monitoring or controlling his activities”.

How many warnings of an attack was the president given?

The warnings weren’t just occasional; they kept coming.

In 1998,1999 and 2000, there were several reports, including one titled Terrorism: Bin Laden Extremists Planning Attacks.

Then in 2001, when George W Bush took over as president from Bill Clinton, the frequency increased, and by the summer the language was increasingly alarming, with one briefing simply titled: Bin Laden Determined To Strike in US.

Did officials know planes could be used in an attack?

Yes, as early as 1998.

A brief dated September 10 that year, titled Bin Ladin’s Location and Intentions, recorded his stated preference for striking Washington, then added that the group behind that summer’s East Africa US embassy bombings “may fly an explosive-packed airplane into a US city”. The Federal Bureau of Investigation (FBI) had quickly determined that those attacks had been carried out by al-Qaeda.

The same memo cautioned that intelligence gave “no conclusive information” on his intentions for future attacks.

That December, another brief described conspirators who “had evaded security checks during a recent trial run at an unidentified New York airport”, and noted that “some members of the Bin Laden network have received hijack training.”

That brief, dated December 4, 1998, was titled Bin Laden Preparing To Hijack US Aircraft and Other Attacks.

Does this prove the US government knew 9/11 was coming?

No. It may raise serious questions about why it did not appear to be more prepared, but it does not show any evidence of a lack of will by the government to prevent an attack.

The newly released material reinforces the explanation that intelligence agencies had many pieces of the puzzle but had failed to assemble them into a clear picture in time.

There is no evidence at all to support conspiracy theories about government involvement or that government officials had prior knowledge of the attacks, which have grown in popularity in the US.

Are all of these documents new?

No, some of the briefs have been made public before.

The two best-known items were already out. The August 6, 2001 brief, Bin Ladin Determined To Strike in US, was declassified in 2004 at the 9/11 Commission’s request.

The broader findings are also known, including that US intelligence had tracked bin Laden’s intentions for years, that aircraft featured in the reporting, and that none of it identified the time, place, method or targets of the eventual plot. The 9/11 Commission set all of this out more than two decades ago.

The release adds the underlying documents, in the briefers’ own words, rather than a commission summary.