(CNN) -- Four Swedish nationals will stand trial in Copenhagen Friday in what officials describe as the most serious ever Islamist terrorist plot in Scandinavia.
The alleged plot, which counter-terrorism officials in the United States and Scandinavia believe was directed by al Qaeda operatives in Pakistan, targeted Jyllands Posten, the Copenhagen-based newspaper responsible for publishing controversial cartoons of the Prophet Mohammed.
The four persons are alleged to have planned a gun attack on the newspaper, followed by "the execution" of hostages.
The cell's alleged plans were thwarted by a joint operation by Swedish and Danish security services, which tracked the alleged terrorists as they drove from Sweden to Denmark in December 2010, with a submachine gun, a silencer, and several dozen 9mm submachine gun cartridges.
The four persons - Mounir Dhahri, 46, of Tunisian descent, Munir Awad, 31, of Lebanese descent, Sahbi Zalouti, 39, of Tunisian descent, and Omar Aboelazm, 32, of Egyptian descent - are charged with plotting to kill a large number of people at the newspaper. They have denied the charges.
Western security services believe the plot was part of a broader al Qaeda conspiracy, authorized by Osama bin Laden, to strike Europe with attacks mirroring Pakistani terrorist group Lashkar e Taiba's Mumbai attack in November 2008, which killed nearly 200 people.
In October 2010, the U.S. State Department issued an unprecedented travel advisory warning Americans of the potential for a similar attack in Europe.
Dhahri, the suspected ringleader of the cell, Awad, and Zalouti had all travelled to Pakistan in early 2010. Awad and Zalouti - travelling separately - were arrested by Pakistani authorities in August 2010 before they could reach North Waziristan and were subsequently deported, according to a Swedish counterterrorism source.
Dhahri evaded capture, and he is believed to have received training in Pakistan and then went to Sweden shortly before the group allegedly began to plot their attack, according to the source.
Awad, the Lebanese-born suspect, had long been on the radar screen of Swedish counter-terrorism services. He was suspected of having joined up with jihadist militants in Somalia in 2006, before fleeing the country when Ethiopian troops launched a military operation against Islamist militants there, according to a Danish security source.
By October 2010, Swedish security services had begun tracking the cell, placing listening devices in their apartments. A Swedish counter-terrorism source told CNN the group only settled on attacking the newspaper shortly before the planned attack, and often squabbled.
On the evening of December 28, 2010, three of the alleged cell members set off from Stockholm in a silver Toyota rental car with Dhahri at the wheel. Security services continuously monitored the progress of their vehicle, including from the air. Zalouti bailed from the journey at the last minute, and was later arrested in Stockholm, according to court documents.
It was just after 2 a.m. when their vehicle crossed the iconic Oresund bridge connecting the two countries. When they reached Copenhagen, they were initially unable to find the address where they planned to sleep. Just after 10 a.m. on December 29, Danish police, concerned the men may be about to try launch their operation, moved in to make the arrests.
Authorities had already taken precautions. When they learned the group was planning to travel down to Denmark to allegedly carry out the operation, they secretly disabled their weapons, according to a Swedish counter-terrorism source.
Plastic wrist strips were also found in their car, according to court documents, and security services said they believed the materials were going to be used to handcuff hostages. Security services believe the plan was to try to take up to 200 journalists hostage at the newspaper and execute many of them, a Swedish counter-terrorism source told CNN.
"Our assessment is that their plan was to try to get access to the Jyllands-Posten building and carry out a Mumbai-style attack," the head of Denmark's intelligence service, Jakob Scharf, said in a press conference after the group was arrested.
Cash amounting to $20,000 was also recovered from the suspects, and a pistol and ammunition were found in one of their apartments, according to court documents.
The trial may shed light on what officials believe are connections between the alleged plotters and al Qaeda terrorists in Pakistan. In total, police made more than a thousand recordings of the plotters' conversations, some of which are expected to be played in court.
A Swedish counter-terrorism source has told CNN that investigations have revealed a complex set of connections between the plotters and a network linked to Ilyas Kashmiri, a senior Pakistani al Qaeda operative who Western intelligence believe orchestrated al Qaeda's plans to hit Europe with Mumbai-style attacks.
Swedish security services established that Awad, the alleged Lebanese-born plotter, moved in the same circle as "Farid," a Stockholm-based militant of Moroccan descent who is suspected of acting as facilitator for Ilyas Kashmiri's terrorist network, according to a Swedish counter-terrorism source. Also involved with Kashmiri's network was David Headley, an American of Pakistani descent who pleaded guilty two years ago to helping plot the Mumbai attacks.
According to an interview of Headley by India's National Investigation Agency that was obtained by CNN, Headley met with Farid in 2009 in relation to a plot Headley himself was planning against the Jyllands Posten newspaper.
The newspaper and its cartoonists has been targeted by several plots in recent years, including a plot by a Norwegian al Qaeda cell broken up in July 2010.
Western counter-terrorism officials say that despite the reported death of Kashmiri in a drone strike in June 2011, al Qaeda has not given up its hope to launch gun and hostage execution attacks in Europe because of the huge publicity and fear such attacks would create.