A series of deadly explosions in Belgium's capital city were likely planned months in advance by a network of Islamic State radicals, two terrorism experts say.

The Islamic State has claimed responsibility for three explosions that left dozens dead at an airport and a subway terminal in Belgium on Tuesday. A posting on an Islamic State news website said its operatives were behind the attacks. The post claimed the attacks were acts of revenge, in response to Belgium's support for an international coalition against the Islamic State.

Former intelligence officer Michel Juneau-Katsuya, of the Northgate Group, and terrorism expert John Thompson, of the Strategic Capital Intelligence Group, both say the attack was likely connected to the arrest of Salah Abdeslam, the man who allegedly planned a series of deadly terror attacks in Paris last November. Abdeslam was captured Friday in Brussels, by a heavily-armed squad of SWAT police officers.

An Iraqi intelligence official told the Associated Press on Tuesday that ISIS had been planning a Brussels attack for May, but Abdeslam's arrest accelerated their plans. The intelligence official spoke on condition of anonymity, and cited sources in the ISIS-held city of Raqqa, in Syria.

Later on Tuesday, investigators found an Islamic State flag and an undetonated explosive device containing nails at a home in the Schaerbeek neighbourhood of Brussels. Several chemical products were also found at the home, Belgian federal prosecutors said.

Thompson said the timing of the attacks may have been spurred by Abdeslam's arrest, but planning such bombings would likely take longer than four days. The arrest could have simply encouraged the terrorists to act sooner, rather than later.

"If this was arranged in four days, that would be a real miracle," Thompson told CTV News Channel on Tuesday. "It takes a lot longer to arrange a terrorist attack, especially with a suicide bomber, than most people realize."

Thompson said it typically takes months to arrange for a suicide bombing, with time needed to scout the location, obtain the bomb materials, and talk the suicide bomber into carrying out his mission. However, those plans may have already been addressed by the time Abdeslam was arrested, Thompson said.

Juneau-Katsuya said retribution is one likely motive, but the bombers may have also been feeling pressure from investigators closing in on them.

"That will be sort of pushing them into action," Juneau-Katsuya told CTV News Channel.

Bomb targets

Thompson said any bombers would have had to construct their vests in the last few days, as homemade explosives tend not to last very long. They're also not very stable, he said.

He added that the bombers likely targeted the airport and the subway because they're good places to maximize the number of casualties.

"These areas have to be open, they have to be accessible and they're often crowded, so you can cause the greatest number of casualties with the least amount of effort," Thompson said.

Suicide bombers are also very difficult to spot in a crowd or stop from carrying out their mission, he said. "Once someone is actually wearing the bomb, it's often very hard to notice them until it's too late."

Thompson said Brussels is likely safe from further attacks in the next few days, as the city deploys police and military personnel to beef up its security. However, that doesn't rule out the possibility of more attacks in other European cities, he said.

"The ISIS network in Europe is large enough that there could be other attacks in other cities very soon," he said.

Backlash and radicalization

Juneau-Katsuya said the attacks will likely continue a "vicious cycle" of anti-Muslim sentiment in Europe. Brussels is home to a relatively large Muslim population, and an estimated 400 individuals from that community have been linked to ISIS activities abroad, he said.

"People will be very suspicious," he said. "That backlash will bring sort of more resentment and anger and frustration coming from that same (Muslim) population that is also victimized now by the general population."

Juneau-Katsuya said those circumstances could drive young Muslims toward radicalization.

With files from The Associated Press