( CNN ) The suspected Islamist terrorists who had a shootout with police on Thursday in Verviers , Belgium , have ties to ISIS-linked cells in other European countries , a senior Belgian counterterrorism source told CNN on Friday . The two suspects who died in the shootout are believed to have fought with ISIS in Syria , the source said . Another man was captured but has n't revealed any information . There also was a dire warning : Not all the terrorists in the cell have been rounded up , according to the source . Belgian police have taken out part of the terror network plotting an attack in Belgium , but they have not yet taken down every component of it , the source said . There is fear others may try to avenge the two men . Authorities investigating the aftermath of the raid in which the men were killed found materials used to make bombs , including chemicals for the powerful explosive TATP , but the suspected terrorists had yet to assemble those weapons . They also had grenades . Cell members had discussed attacking police in wiretapped phone conversations , but Belgian investigators still did not know what they were intending to target for sure , the source said . The discovery of police uniforms after the raid suggested the possibility they may have been trying to gain access to sensitive sites . The source characterized the cell as one with organized structures , a logistical support network and links back to Syria and Iraq . Belgium is one of the nations participating in airstrikes in Iraq , and officials are concerned this plot is ISIS `` pivoting '' to launch attacks on the European countries involved in the military action against their troops in Iraq . `` I think one of the things that 's happening is we 're seeing something that 's surprising me . That is the pivot of ISIS , '' Philip Mudd . a former CIA counterterrorism official and a CNN analyst , said . `` Just last summer , we were talking about how quickly ISIS was taking over geography . For a terrorist group to turn around in six or eight months and start training people for operations in Europe is remarkable . '' In the raid in Verviers , a city of about 56,000 people , police approached three suspects as they carried large duffel bags outside of a former bakery that might have been their lair , a Western intelligence source told CNN . The suspects immediately opened fire with multiple weapons , prosecutors ' spokesman Thierry Werts said . Police returned fire ; two gunmen were killed and another was arrested , federal prosecutor Eric van der Sypt said . That raid turned up four Kalashnikovs , handguns , bomb-making materials and police uniforms , van der Sypt said . Van der Sypt refused to name the two killed in Verviers , but said `` we have a pretty good idea '' who they were . Answering reporters ' questions , the prosecutor would not comment on reported plans by the suspects to abduct or behead any victims . The Belgian counterterrorism source said Belgian authorities had asked the CIA for help in finding the director of the plot , a Belgian who was in Greece . Neither the CIA nor Greek officials were able to locate the man . He is described as a key link to ISIS in Syria for the terror cell and once fought there . The two men killed in the shootout were among more than a dozen people rounded up in across-the-country raids designed to stop a group 's allegedly imminent attack -- a plot to kill Belgian police in streets and stations -- van der Sypt said earlier Friday . The alleged plotters -- including the two people killed Thursday night in a battle with police in Verviers -- were confronted in 12 raids across Belgium from Thursday into Friday , Sypt said . `` Could have been hours , certainly no more than a day or two '' before terror attacks were to begin throughout Belgium , van der Sypt told reporters of the alleged plot . Besides the two killed , 17 have been arrested in the investigation -- 13 in the Belgium raids and four others in France , including two who were detained while trying to cross from France into Italy , van der Sypt 's office said . Investigators have n't found any links between the alleged Belgium plot and last week 's Islamist terror attacks that killed 17 people in Paris , the prosecutor said . But the Belgium raids come amid fears of ongoing terror threats in Europe -- as many as 20 terror cells with 120 to 180 people may be ready to strike in France , Germany , Belgium and the Netherlands , a Western intelligence source told CNN . Thursday night 's raids were a dramatic culmination in a chain of Belgian police investigations into an alleged terror cell that included people who had fought in Syria , van der Sypt said . The U.S. intelligence community knew for weeks of the plot in Belgium and was sharing critical information with Belgian authorities , several U.S. officials told CNN . The officials would n't say precisely what they knew , how much they knew , and when they knew it . There is concern those details could signal other militants planning other potential attacks . One U.S. official told CNN 's Barbara Starr the entire `` developing plot was being monitored and watched . '' The official said , `` We were aware . We were tracking this , '' and added , `` 'There is a high probability other attacks were being planned '' before the plots were disrupted by the raids . A European security source told CNN that Belgian authorities arrested two men returning from Syria over the weekend and interrogated them , then decided to act quickly . The United States was aware of the timing of the Belgian moves , U.S. officials said . Verviers is a little off the beaten path in eastern Belgium , the last stop before open fields and forest . Like many European towns , it 's densely built . People live close together . The whole city must have heard the explosions and the gunfight , according to resident Frederic Hausman . `` I can hear it . Everybody can hear it , '' he said . `` In this little city , everybody heard the sound . '' From his window , he could see police firing assault rifles at a nearby house . Hausman recorded the assault on the house and posted it on YouTube . The two who were killed were of North African descent , the Western intelligence source said . Verviers is home to many people with Moroccan roots , according to a study by the nearby University of Liege . Immigrants make up more than 11 % of the population , with the largest contingent from other European countries . Moroccans are the next-largest group . Verviers is about 69 miles ( 111 kilometers ) east-southeast of Brussels and 200 miles ( 322 kilometers ) northeast of Paris . The raids came as authorities monitored people returning from Syria , said Werts , the prosecutors ' spokesman . Police had arrested , questioned and searched Neetin Karasular , a Belgian suspected arms dealer allegedly aligned with ISIS and suspected of providing weapons to Amedy Coulibaly , the man who attacked a Paris kosher supermarket . Karasular knew Coulibaly 's wife , Hayat Boumeddiene , who is also a terror suspect in France . Karasular 's lawyer Michel Bouchat said his client was facing local firearms charges and had no connection with any jihadi groups or terror plans . In the process , police turned up names that solidified their suspicions about known persons , the Western intelligence source said . Last weekend , they arrested two more men at the Charleroi airport -- as they returned from Syria -- squeezed them for information then decided to act quickly , the source said . `` The investigation made it possible to determine that the group was about to carry out major terrorist attacks in Belgium imminently , '' Belgian prosecutor spokesman Werts said . Authorities believed the suspects in Thursday 's gun battle had been providing documents and weapons to men returning from Syria , the intelligence source said . A senior Belgian counterterrorism official told CNN that the alleged terror cell is believed to have received instructions from ISIS . Belgium is putting 150 troops on standby for deployment in light of the increase in the country 's terror threat level , Belgian Interior Minister Jan Jambon said Friday . It has not been determined where they will be deployed , he said . The threat level will stay at three ( with four being the highest ) for one month , Prime Minister Charles Michel told CNN affiliate RTL . `` It is a way to mobilize all forces to raise security , '' added . In France , police also made at least 12 arrests Friday in the probe of last week 's slayings of 17 people . Additionally , two people suspected of involvement with the alleged Verviers suspects were detained while trying to cross from France into Italy through the Frejus tunnel , a spokesman for Belgium 's federal prosecutor 's office said Friday . Belgium is asking France to extradite those two , said the spokesman , who would not disclose the suspects ' names or nationalities . In neighboring Germany , police in at least two cities arrested men they accuse of supporting jihadis in Syria . These suspects did not appear to be planning homegrown attacks , German authorities said . The fear of terror was already high in Western Europe after the bloodbaths in France that followed multiple threats of attacks from ISIS -- the Islamist group fighting to establish what it calls its Islamic caliphate in Iraq and Syria -- and al Qaeda in the Arabian Peninsula . AQAP has claimed responsibility for the Paris slayings . Men claiming to be ISIS terrorists and speaking French promised new attacks in France , Belgium , Germany and Switzerland , in an online video found on Thursday . ISIS has said it would lash out at European countries participating in bombardments in the Middle East . More than 3,000 Europeans have left to fight in Syria in recent years . Authorities have long warned that those fighters could return and carry out attacks at home . British intelligence has also warned another Islamist group in Syria was planning `` mass casualty attacks against the West , '' an apparent reference to the Khorasan group . The Belgian raids came against the background of a terror trial of dozens of men suspected of recruiting jihadists or trying to go to Syria to fight . An Antwerp court was to return verdict this week but postponed it due to the Paris attacks . CNN 's Mariano Castillo , Richard Greene , Margot Haddad , Catherine E. Shoichet , Dheepthika Laurent and Jim Sciutto contributed to this report .
Officials believe this is a new strategy from ISIS to organize attacks on nations in airstrike coalition . Counterterrorism official tells CNN that terrorists killed in Belgium raid had ties to ISIS . U.S. sources tell CNN that they knew of planned attacks and shared information with Belgians .