Despite high security measures already in place following the Paris attacks in November and a flurry of police activity in recent days, attackers managed to detonate three explosive devices across Brussels during Tuesday morning’s rush hour there.

Two of the blasts were in Brussels Airport in Zaventem, while one was underground in the Maelbeek metro station in the heart of the city. The Maelbeek metro station is near both the European parliament and commission. More than 30 people were killed and more than 100 were wounded. It is unclear what type of explosive devices was used, though reports indicate that at least one of the explosions was carried out by a suicide bomber.

The attacks, in a city that is home to NATO and a host of European nongovernmental organizations, come after months of threats, alerts and high-profile police raids that have sought to disrupt Islamic State plots to strike across Europe.

“What we’ve feared has happened; we were hit by unforeseen attacks,” Belgian Prime Minister Charles Michel said at a news conference.

Following the Paris attacks in November, in which Islamic State gunmen killed 130 people in a series of mass shootings and suicide attacks, Brussels raised its terror threat level to its highest possible alert and went on lockdown, as police and intelligence agencies traced some of the Paris attackers’ origins back to the Molenbeek neighborhood of Brussels. Roughly a week later, the Belgian government lowered the threat level from “serious and imminent” to “possible and likely,” where it had remained until Tuesday’s attacks. It is now back to its highest level.

“This is the ultimate paradox in counterterrorism,” said Bruce Hoffman, the director of Georgetown University’s Security Studies Program. “You can’t protect every target, everywhere, all the time.”

“They’ve been on complete alert, and still all these measures are still insufficient against a determined adversary,” he added.

Tuesday’s attacks hit a city where soldiers, clad in body army and Fabrique Nationale carbines, were already stationed at metro entrances, government buildings and various other locations throughout the city. The soldiers often patrol in pairs and are occasionally easily marked by their large Mercedes transport trucks.

The blasts tore through the departures area at Brussels airport, which is regularly patrolled by soldiers but does not have a screening area for passengers. After exiting the metro or inter-city trains arriving at the airport, passengers walk to an atrium where a three-level elevator takes them either to the departures or arrivals area. To enter the departures area, passengers only have to scan their boarding passes before proceeding to security. At rush hour, these areas, including the ticketing desks, can easily back up, something the attackers clearly took advantage of as one of the bombers appeared to have detonated a device in exactly that vicinity.

In the Brussels metro however, the only security is the occasional police or group of soldiers at various entry points.

“This is what makes planning for these attacks so difficult,” Hoffman said. “You might place security at the central train station, but there are still a cornucopia of different areas that are open to attack across the city.”

In 2011, a suicide bombing carried out by Islamist militants at the Domodedovo International Airport bombing used similar tactics, striking in the arrivals hall where there were no security checkpoints.

The explosions in Brussels comes just days after Belgian police arrested Salah Abdeslam, suspected of involvement in the Paris attacks. Police have said the 26-year-old planned logistics for the cell that carried out the deadly siege in November and may have been a key leader in the group. According to Hoffman, the attacks Tuesday, just four days following Abdeslam’s arrest, indicate how entrenched his compatriots are in the region.

“Their leader was picked up and the network didn’t crumble. They adjusted their plans and responded in in a pretty expeditious fashion,” Hoffman said, though it is currently unclear if Abdeslam was directly connected to Tuesday’s blasts. “It indicates their network and infrastructure is much deeper than people have assumed.”