Habib Marwan attacks

Throughout Day 4, the United States was the target of a coordinated series of terrorist attacks masterminded by Habib Marwan. These included the bombing of a commuter train in Santa Clarita, the kidnapping and trial of Secretary of Defense James Heller, the attempted melting down of six nuclear power plants, the downing of Air Force One, and culminated in the unsuccessful nuclear strike on Los Angeles. The attacks were carried out by a large network of independent sleeper cells, each reporting to Marwan himself. Although the majority of the attacks were thwarted by American military and law enforcement personnel, over 123 were left dead in their wake. Additionally, numerous civilians and federal agents were killed in the lead-ups to the attacks.

The attacks also had major political ramifications, including the ascension of Charles Logan to the presidency, increased tensions between the U.S. and China, and Jack Bauer's faking his own death.

Motives
In a prerecorded video intended for release to the public, Habib Marwan stated that the attacks were a response to America's interventionist and "imperialist" foreign policy as well as its people's apathy toward such. Marwan also stated that James Heller's kidnapping was designed to instill fear into the American people of letting their leaders appear in public.

Planning


The planning for the attacks dated back to nearly five years before Day 4, which was approximately when the Araz family moved to the United States.

Dobson Override
Habib Marwan, a longtime employee of McLennen-Forster, was the principal engineer of the Dobson Override, a device commissioned by the Department of Defense. It was designed to remotely take control of a nuclear power plant and shut down its reactor in the event of an emergency. However, Marwan planned to do the opposite with it, which was to take control of every plant in the U.S. and cause its core to melt down, releasing nuclear material into the environment and killing millions.

Merely being in possession of the Override would not be enough to accomplish this; each individual plant was protected by a firewall, and hacking through them all would require massive computer power. So Marwan planted a sleeper agent, Ali, into the IDS Data Systems corporation. He would then use the company's servers to upload viruses to the reactors' firewalls, leaving them vulnerable to manipulation by the Override. In order to prevent the Nuclear Regulatory Commission from detecting such an attack, Marwan needed a means of disguising it. Before the attack on the plants was to commence, Marwan would have one of his cells abduct Secretary of Defense James Heller. Heller would be put on trial for "war crimes" and executed in front of a live feed. This feed would be streamed by millions of computers across the world, serving as a "Trojan horse" for distributing the viruses.

Nuclear warhead
This would only be the first part of Marwan's planned wave of terror; his true endgame was to obtain a nuclear warhead and deliver it, by missile, to a major American city. To do this, he would need Section 3 of the Red Chapter from the Nuclear football, which contained the locations and activation codes of all U.S. nuclear warheads.

In order to obtain the Football, Marwan recruited former Air Force pilot turned mercenary Mitch Anderson into his conspiracy. Anderson's mission was to impersonate a USAF pilot, hijack a stealth fighter, and use it to shoot down Air Force One, which was scheduled to land in Los Angeles at 11pm on Day 4. A team, tracking the Football with a transponder embedded in its case, would then move in and retrieve it from the plane's wreckage. In preparation for his mission, Anderson had been training with fellow terrorist Aziz to fly an F-117 Nighthawk, using a flight simulator on his computer.

Final preparations
Three months before Day 4, Tomas Sherek, who would be responsible for overseeing the theft of the Override and James Heller's kidnapping, entered the United States through Mexico, killing the men who helped him cross the border in the process.

For his plan to kidnap James Heller to succeed, Habib Marwan needed to utilize someone close to the Secretary. Marwan tasked two of his operatives, Mandy and Gary, with deceiving Richard Heller into giving away his father's whereabouts.

On the Tuesday before Day 4, at approximately 4am, Richard met the two operatives—posing as average partygoers—at a bar. One of them approached Heller and asked if he wanted to "party." Richard then drove the couple back to his house, where they became intoxicated. Heller began having a one-night stand with Gary, and Mandy used this as an opportunity to place a bug on the former's cellphone, enabling the terrorists to listen in on his calls. They would eventually find out that the Secretary would be paying an off-the-books visit to his son's house at 8am; this was when they planned to execute the kidnapping.

Train bombing and Override theft
By the beginning of Day 4, Marwan was ready to initiate the first stage of his operation: to steal the Dobson Override. Using his position at McLennen-Forster, he learned that a courier for the company, Brody, was transporting the Override by way of a commuter train. The leader of one of Marwan's sub-cells, Navi Araz, tasked Tomas Sherek with stealing the device while it was in transit. Sherek and his subordinate Dar planned to derail the train via explosives and retrieve the device from the wreckage.

In Santa Clarita, Dar parked a truck carrying C-4 on a remote railway, creating a collision course with the train. At 7am, the commuter train collided with the truck, resulting in a massive explosion. The train was thrown off the tracks, killing over 32 passengers and injuring many others. Dar, watching the collision from a safe distance, then motorcycled to the crash site. After searching through the wreckage, he found Brody—grievously injured—with the Override handcuffed to his wrist. Dar executed the courier, took the device, and delivered it to Navi Araz. With his assignment complete, Sherek met with Maurice Landstrass, who was to help him escape the country.

CTU Los Angeles was alerted to Tomas Sherek's presence in the U.S., and sent a field team to pick him up. They also picked up chatter of a possible attack on the West Coast at 8am, and mistakenly believed it to be referring to the train bombing that had just happened. However, after being tortured by Jack Bauer, Sherek revealed his primary objective: the Secretary of Defense.