(CNN) -- German federal police on Friday arrested three men suspected of belonging to al Qaeda, a counterterrorism official told CNN.
The German official told CNN that arrests were made in Dusseldorf and Essen, but he did not provide more details.
The three are expected to appear in court in Karlsruhe on Saturday. No details of impending charges are known, but the federal prosecutor's office has scheduled a Saturday news conference.
Several German news outlets reported that chemicals useful in the production of explosives were found on the suspects and that those arrested were suspected of "plotting a significant attack."
The German magazine Der Spiegel reported that one member of the group -- identified as Abdeladim K., a German resident of Moroccan descent -- is believed to have received training overseas and to have been in frequent online communication with a senior al Qaeda operative in the Afghanistan-Pakistan border region in the lead-up to his arrest. According to the magazine, the suspects were arrested after German authorities intercepted communications referring to a "test" in a several-months-long investigation that also saw the involvement of the CIA.
The arrests "succeeded in averting a concrete and imminent danger, presented by international terrorism," German Interior Minister Hans-Peter Friedrich said in a statement. They also showed "Germany remains a target of international terrorists."
German authorities have not indicated whether they believe the group had selected a specific target, though German media outlets noted that the Eurovision song contest, a pan-European televised national musical talent contest, is being held in Dusseldorf in mid May.
The arrests come at a time of heightened concern in Germany over the threat of al Qaeda terrorism.
Last November, Germany raised its alert level after intelligence suggested al Qaeda was plotting a "Mumbai-style" attack against the country as part of a broader conspiracy against European cities.
Islamic militants staged assaults against several targets in the Indian city of Mumbai in November 2008.
A German intelligence official told CNN Friday that while concerns had eased over the plot, security forces remained vigilant.
In September 2007, German authorities broke up a plot to attack American servicemen in the country by a German terrorist cell trained in bomb-making by the Islamic Jihad Union -- an al Qaeda-affiliated Uzbek group based in the tribal areas of Pakistan.
The four plotters, who were convicted of the plot in a trial held in Dusseldorf last year, had amassed 100 times more peroxide-based explosives than had the bombers of the London subway in 2005.
German media outlets have reported that the plot allegedly foiled in Germany Friday was on a similar scale to the 2007 plot.
German intelligence officials have grown increasingly concerned about the numbers of German Islamist extremists traveling for jihad in the Afghanistan-Pakistan border region.
While most are believed to travel there to fight in Afghanistan, their presence in the tribal areas of Pakistan has provided al Qaeda and affiliated groups with opportunities to recruit operatives for plots against the West.
German intelligence officials said several militants from the German port city of Hamburg who travelled to train in Pakistan's tribal areas were involved in an al Qaeda attack plans against Europe last fall.
According to German officials, information from one of them -- Ahmed Sidiqi -- helped prompt an unprecedented U.S. State Department travel advisory for Americans travelling in Europe. Sidiqi subsequently provided useful intelligence to German counterterrorism agencies, according to German intelligence officials
According to German authorities, more than 200 Germans have traveled to training camps in Pakistan in recent years. German intelligence officials told CNN that more than 40 are still believed to be fighting and training in the Afghanistan-Pakistan border region.
The arrests come just two weeks after one of the Germans currently thought to be in the tribal areas purportedly released a message calling for attacks in Germany. The message came from Mounir Chouka, an extremist who had lived in Bonn.
According to a translation of the message by the SITE Intelligence Group, Chouka urged attacks in Germany following what he called a "pyramid system," in which priority should be given to targeting the head of state, then federal officials and soldiers, and finally average citizens.
intelligence officials said Chouka is one of two brothers who have emerged as German spokesmen for the al Qaeda-affiliated Islamic Movement of Uzbekistan (IMU). The brothers are suspected of facilitating the recruitment of dozens of Germans into the IMU, including Ahmed Sidiqi's Hamburg group.
Of greatest concern however are the 100 or so German jihadists who have returned home. According to a German intelligence source, many of them are under observation by German intelligence agencies because of their continued support for extremist Islamist views and their continued communication with al Qaeda-linked militants in Pakistan.
Homegrown radicalization has emerged as a significant problem in Germany in recent years, fuelled according to German intelligence officials by the rise of social media and the emergence of German-language jihadist sites that have called on German Muslims to fight American troops in Afghanistan.
In March, two American servicemen were killed when a German Islamist extremist boarded their bus at Frankfurt airport and shot them at point-blank range. The alleged shooter, Arid Uka, was Facebook friends with several prominent German Islamist extremists, according to German intelligence officials.