(CNN) -- A single phone call by Osama bin Laden's trusted courier tipped off U.S. officials to his Pakistan compound, ultimately leading to the raid that killed the al Qaeda leader, a senior Pakistani intelligence official told CNN Saturday.
The telephone call the courier made was "not the final one -- it was the initial piece of evidence" that sparked the focus on the compound in Abbottabad, the official said.
Four years ago, U.S. officials uncovered the identity of a trusted bin Laden courier -- later identified as a Kuwaiti named Abu Ahmad -- whom they believed was living with and protecting the al Qaeda leader.
The Washington Post, citing U.S. intelligence officials, reported Friday that Americans had intercepted a "catch-up phone call" Ahmad took from an old friend.
"This is where you start the movie about the hunt for bin Laden," one U.S. official briefed on the intelligence-gathering leading up to the early Monday raid on the compound told the Washington Post.
The Pakistani official told CNN the phone call was made by the courier, though he didn't know when.
The courier and his brother were among those killed in Monday's raid. In recent days, the materials taken from bin Laden's compound continued to yield a trove of intelligence, including details about a possible attack on the 10th anniversary of 9/11.
At a briefing Saturday focusing on the intelligence aspects of the raid, U.S. officials released five videos showing the al Qaeda leader. They were seized at his compound.
One video looks like a home movie, a portrait of a graying man watching television, but it is an image that suggests how conscious bin Laden was of his image. The other videos showed him in more formal attire, making remarks, but U.S. officials removed the audio in all five videos.
As early as February 2010, al Qaeda members discussed a plan to derail trains in the United States by placing obstructions on tracks over bridges and valleys, the alert said, according to one law enforcement official.
The plan was to be executed later this year, coinciding with the 10th anniversary of the September 11 attacks, though no specific rail system was identified, the official said.
The Department of Homeland Security confirmed a notice was sent to federal, state, local and tribal authorities.
"We have no information of any imminent terrorist threat to the U.S. rail sector, but wanted to make sure our partners are aware of the alleged plotting. It is unclear if any further planning has been conducted since February of last year," spokesman Matt Chandler said.
Rail agencies across the United States heightened security.
A U.S. official said that "valuable information has been gleaned already" from the information gathered at bin Laden's compound, though no specific plots or terrorist suspects were identified.
But the material suggests that al Qaeda was particularly interested in striking Washington, New York, Los Angeles and Chicago, according to the law enforcement official who spoke on condition of anonymity.
U.S. authorities have found that al Qaeda appeared especially interested in striking on significant dates like July 4, Christmas and the opening day of the United Nations.
The material seized from the compound included audio and video equipment, suggesting bin Laden may have taped messages there, a U.S. official said.
Ten hard drives, five computers and more than 100 storage devices, such as disks and thumb drives, were also found, a senior U.S. official told CNN.
Dozens of people in Abbottabad have been arrested because of their suspected connections to the compound where Osama bin Laden was shot and killed, a Pakistani intelligence official said Friday. Investigators want to know whether any of the people are al Qaeda members or sympathizers.
The United States and Pakistan have been allies for years in the anti-terrorism effort, but U.S. concerns that Pakistanis haven't been robust enough in the fight against Islamic militants and suspected U.S. drone attacks that killed innocent civilians have heightened tensions. Another suspected drone strike killed 12 suspected militants on Friday in the Pakistani tribal region.
Questions remain over why and how Pakistani intelligence officials could not have known bin Laden was hiding out in the city, which is home to a military academy and has a strong military presence.
Pakistani armed forces chiefs issued a statement Thursday admitting "shortcomings in developing intelligence" on the terrorist leader's presence in the country.
The army chief of staff, Gen. Ashfaq Parvez Kayani, "made it very clear that any similar action, violating the sovereignty of Pakistan, will warrant a review on the level of military/intelligence cooperation with the United States," the statement said.
Since the raid, Pakistan has ordered U.S. military personnel on its territory drawn down to the "minimum essential" level, the statement said.
CNN's Nick Paton Walsh contributed to this report