Hamburg, Germany (CNN) -- Al Qaeda is still planning Mumbai-style attacks in Europe, with the United States also possibly being targeted, counter-terrorism officials in Europe and the United States tell CNN.
The discovery of al Qaeda's plans to launch coordinated attacks in several cities in Britain, Germany, and France led to the U.S. issuing an unprecedented travel advisory in October for its citizens traveling in Europe.
European counter-terrorism officials tell CNN they believe the aim was to carry out the attacks before the end of this year. The expected timeframe of the plot had not previously been disclosed.
In November 2008 gunmen belonging to Lashkar e Taiba, a Pakistani Jihadist group affiliated with al Qaeda, went on a shooting rampage against several targets in Mumbai, including its most prestigious hotel, the main railway station and a Jewish center, killing more than 160 people.
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In an exclusive interview with CNN, Dr. August Hanning, a former head of Germany's foreign intelligence service, said intelligence indicated that al Qaeda had already started planning to launch Mumbai-style attacks in the United States.
"We have got information that they have planned or are planning a plot like the Mumbai plot in Europe and the United States," said Hanning who retired late last year as State Secretary in Germany's Interior Ministry, one of the country's most senior counter-terrorism positions.
The revelation is the most concrete indication yet that al Qaeda is planning mass casualty gun attacks on U.S. soil.
A senior U.S. counter-terrorism official told CNN that U.S. intelligence agencies have for some time been concerned that al Qaeda would attempt to replicate aspects of the 2008 Mumbai attack on US soil. "The assumption has been that they would make plans to do this and the potential threat is being treated very seriously," the official told CNN.
The capture of Ahmed Sidiqi, a militant from the German port city of Hamburg, in Afghanistan in July, helped Western intelligence uncover the conspiracy, according to European and U.S. counter-terrorism officials. Sidiqi is currently being held in American custody at Bagram air force base in Afghanistan.
Information came from "different sources ... and this is one of the sources," Hanning told CNN. His statement was echoed by a senior U.S. counter-terrorism official.
Western intelligence agencies also learned that Ilyas Kashmiri, a senior al Qaeda operative, had a planning role in the plot. According to U.S. counter-terrorism officials, Osama bin Laden himself signed off on the plot.
Kashmiri, a veteran jihadist who made his name fighting Indian troops in the Kashmir conflict, has in the last year emerged as a key planner of al Qaeda operations against the West, according to Western officials and court documents.
Last month Britain's Daily Telegraph newspaper citing intelligence sources reported that Kashmiri met with Sidiqi in Pakistan's tribal areas and boasted that he had already dispatched terrorist teams to Britain and Germany to launch Mumbai-style attacks.
"[Kashmiri] knows our situation in Germany and therefore he is dangerous," Hanning told CNN.
German authorities may have particular cause for concern. German authorities are investigating the alleged involvement of several militants from Hamburg, including Bagram detainee Sidiqi, in the al Qaeda plot against Europe.
Sidiqi and 10 other militants from Hamburg set off for the tribal areas of Pakistan in March 2009, according to German intelligence officials. "When they left Hamburg they [had] decided to join the jihad in Afghanistan or Pakistan but then they came into contact to certain groups then after this they developed the plan ... not to stay there and fight there but to go back and commit some crimes in Germany in Europe," Dr. Manfred Murck, Hamburg's Intelligence chief told CNN in an exclusive interview.
According to European counter-terrorism officials, Sidiqi revealed that four other members of his group were part of al Qaeda's plans to attack Europe. Several of them met with Younes al Mauritani, a senior al Qaeda operative who tasked some of them to return to Europe to prepare the attack, according to the officials.
"The general assumption would be that Sidiqi and some others planned to come back to Germany and might develop terrorist attacks in the long term," Murck told CNN, "this is the general assumption that we do have, but it's not concrete, we don't think they had a concrete plan."
Murck said Hamburg's intelligence agency has found it difficult to untangle how the Hamburg group fitted into Al Qaeda's plans because they have had no direct access to him in Afghanistan. "As far as we can see we don't have the evidence that [theirs] was a terrorist attack in the Mumbai style," Murck stated.
In early October two members of the Hamburg group -- Naamen Meziche and Shahab Dashti -- were reported killed in a drone strike in North Waziristan, one of Pakistan's tribal territories. According to European intelligence officials the group's travel coordinator -- Asadullah Muslih -- is still believed at large somewhere in Pakistan. Murck said his intelligence agency has evidence that Dashti was killed but has not been able to verify the reported death of Meziche.
Rami Makanesi -- another member of the Hamburg travel group allegedly implicated by Sidiqi -- is currently in custody in southern Germany. He is being investigated for membership of a terrorist group but has not been formally charged by German authorities. "He wanted to go to the German embassy or consulate [in Islamabad] and then he was picked up," Murck told CNN.
Murck hinted that some of the Hamburg group may have wanted to return to Europe because they were fed up with conditions in the al Qaeda camps in Pakistan.
"It's not the nice romantic jihad they were thinking about," he said.
According to German intelligence officials, the Hamburg group were recruited by Meziche, the group's ringleader in the Taiba mosque in Hamburg , a mosque -- previously called Al Quds -- attended by 9/11 lead hijacker Mohammed Atta in the late 1990s. In August this year Hamburg authorities closed down the Taiba mosque because of its ties to extremists.
Murck told CNN that 15 foreign radical extremists were deported from Germany based on information authorities collected at the Taiba mosque. But over time he said, more and more clusters of radical extremists formed in the mosque.
"If there is one place, from Denmark even to the United States, where people know if you want to be a brother in the name of Allah and have an idea to be a member of jihad then go to al Quds mosque in Hamburg. It was that famous, and this was one of the reasons that we decided to close it."
Hamburg authorities had to fight a tough legal battle to close the mosque. "We have a Constitution and churches, mosques are protected by our Constitution and it's very difficult for German authorities to forbid praying in such kinds of mosques," August Hanning told CNN.
Hamburg intelligence officials stress that Hamburg is not unique among European cities grappling with the problem of violent Islamist extremism.
"We count about 40 persons at the moment ... who justify violence and find it's right that there is an international jihad ... and that terrorism might be right, and there might be a 100 more that are in close contact to them," Murck told CNN.
"Taken altogether we don't have a real chance to look at each of those 40 or 140, 24 hours a day, every week so what we have to do is to look at the [radical] scene, to have some human sources within that scene."
Radicalization is on the rise in Germany according to German counter-terrorism officials with hotspots emerging in such cities as Berlin, Bonn, Ulm, Frankfurt, Cologne and Hamburg, fueled by radicals' exploitation of online social media sites.
According to Hanning, around 100 to 200 hard cores supporters of al Qaeda in Germany currently pose the greatest concern.
The trajectory that has most worried German counter-terrorism officials is Germans who have gone overseas for terrorism training and returned.
"Our estimate is 220 people who have left Germany for training purposes in Pakistan, being trained in terrorist techniques and nearly half of them have come back to Germany and that has been the real threat for us. ... We know that they still have contact with these dangerous groups in Pakistan," Hanning told CNN.
Murck, Hamburg's Intelligence Chief, says the city's intelligence agencies are determined to do everything they can to prevent a terrorist attack on the city. "We just have to live with the possibility it might happen and with our responsibility to hinder it."