European officials were working feverishly on numerous fronts Tuesday to stave off potential terror attacks while chasing leads from the latest one.
Police raids in Europe thus far have failed to capture the primary person-of-interest in the Paris terror attacks: Salah Abdeslam.
With 26-year-old Abdeslam on the run, Belgian officials left their capital city of Brussels under lockdown for a fourth day amid fears of an "imminent" threat of attack. Prime Minister Charles Michel said Brussels would remain on the highest level of alert for at least another week.
"We are very alert and call for caution," Michel said late Monday. "The potential targets remain the same; shopping centers and shopping streets and public transport."
The metropolitan area of Brussels has a population of about 1.9 million.
Belgium has been central to the probe into the Nov. 13 terror attacks which killed 130 people in Paris. Several of the terrorists had ties to the small European nation, and Abdeslam is believed to have fled there following the deadly siege on the French capital.
One person has been charged by Belgium for participating in the Paris terror attack, though the individual has not been named.
Amid the ongoing manhunt for Abdeslam, the French investigation into attacks has continued apace. One new potential clue emerged overnight with the discovery of an explosive belt in the Paris suburb of Montrouge on Monday.
The find triggered speculation that maybe the belt had malfunctioned or — if it belonged to Abdeslam — that he had aborted a suicide bombing mission.
The identities and ties of alleged terrorists killed in the French police raid on an apartment in Saint-Denis also has been fodder for the probe.
Abdelhamid Abaaoud — the alleged mastermind of the Paris attacks — died in the massive and lengthy operation by French security forces, as did 26-year-old Hasna Aitboulahcen.
Aitboulahcen's family described her as troubled, telling NBC News that she had most likely been "manipulated" by the terror mastermind.
The landlord of the apartment involved in the raid, meanwhile, has been in police custody for days.
Jawad Bendaoud — who told French television he didn't know the people he'd let stay in his apartment were terrorists — was expected to appear before a French judge on Tuesday, according to The Associated Press. Under French law, he must either be formally charged or released by Tuesday.
Outside of the immediate investigation officials were working tirelessly to prevent future attacks.
The State Department also issued a worldwide travel alert citing "increased terroristic threats" from ISIS and other groups, and France's transport minister announced security gates would be installed for cross-border trains. Segolene Royale told French radio that the government was also looking at additional ways to reinforce security at stations which receive international trains.
Meanwhile, French President Francois Hollande was due in Washington on Tuesday to meet with President Barack Obama over how best to defeat ISIS.
Ahead of the meeting, France's ministry of defense confirmed it launched further airstrikes on the ISIS stronghold of Raqqa in Syria.