BRUSSELS -- Two of the men who launched deadly suicide bombings in Brussels on Tuesday, killing 31 people and injuring hundreds, were on U.S. terror watch lists prior to the attacks.
It's still not clear how long the men behind the attacks had been planning them, but it's becoming more apparent that the arrest of Salah Abdeslam may have been the trigger.
The only survivor of the ten attackers who struck Paris in November, Abdeslam was captured Friday in police shootout.
But his attorney, Sven Mary, told CBS News that Belgian police had interrogated him only once over the weekend -- despite his links to both the Paris and Belgium attacks.
"You know, he was interviewed from the first time Saturday. He didn't see any police officers before the attacks. So he saw police officers after the attacks," Mary said Thursday.
And other opportunities missed are also emerging.
The Belgian prosecutor said Wednesday two of the suicide bombers, Ibrahim El Bakraoui and his brother Khalid, had no previously known links to terrorism until March.
But in fact, Khalid had an Interpol red-level arrest warrant for terrorism issued in December. The arrest warrant was issued when the Belgian police connected him to one of the hideout apartments used in the Paris attacks.
Meanwhile, his older brother Ibrahim was arrested as a suspected terrorist fighter by Turkish security forces when he was caught trying to sneak into Syria last summer. When the Belgians didn't take custody of Ibrahim, the Turks deported him to the Netherlands.
Terrorism expert Claude Moniquet told CBS News intelligence agencies have to up their game, and fast. He estimated there may be "dozens" of potential suicide bombers in Europe.
Moniquet said the fear of more attacks is what keeps him up at night.
"The idea that my phone will ring at one moment in the night and someone will tell me there was an attack in Brussels, Athens, London," he said. "I don't know where, but there will be an attack."
Police are still on the hunt for one suspect in connection with the Brussels attacks but also said Thursday there may have been a second bomber in the subway. They're not sure whether he died in the bombing or whether he's on the run.