Investigators believe the cell accelerated the plan when Belgian police discovered Abdeslam's hideout in the Forest district of Brussels last week. The official said one of the two brothers in the Brussels strikes, Khalid and Ibrahim El Bakraoui, rented the Forest safe house that Abdeslam used in the wake of November's Paris attacks
-- definitively connecting the Paris and Brussels attack cells, the official said.
Abdeslam, the most wanted man in Europe, was captured Friday in the Brussels suburb of Molenbeek
. He was the only one at large of the 10 men accused in the Paris attacks.
The Belgian federal prosecutor revealed Wednesday that Brussels attacker Ibrahim El Bakraoui left behind a will found in a laptop computer near a bomb factory raided Tuesday in the city's Schaerbeek district. In the documents, Bakraoui stated he "needs to rush" and "no longer feels safe." He also said that if he takes too much time, he would end up "next to him in prison" -- an apparent reference to Abdeslam the Belgian investigators believe, the official told CNN.
When police arrived at the Forest apartment on March 15, they had intelligence linking it to the Paris attacks but expected the property to be empty. Instead, three terrorists inside opened fire, officials said. A senior Algerian ISIS operative named Mohamed Belkaid provided cover fire, while Abdeslam and another accomplice fled the building through the roofs. A Belgian police sniper later killed Belkaid.
Belgian investigators believe Belkaid was probably the commander of the ISIS terrorist cell behind the Paris and Brussels attacks, the senior official told CNN. When police searched the Forest residence, they found an ISIS flag, a Kalashnikov rifle
and detonators, raising fears the men inside were planning an attack.
But Abdeslam and his accomplice then made a critical error, Belgian investigators told CNN. They phoned an associate whom Belgian security services were monitoring, allowing police to move in and arrest the duo in an operation Friday in Molenbeek.
Belgian security services had taken out one node of the network, but the other node already was accelerating its plans, fearing that Belgian security services were closing in.
During the night of the Paris attacks on November 13, Belkaid and a Belgian ISIS operative identified as Najim Laachraoui
coordinated those strikes in real time from Brussels, officials have told CNN.
CNN also has reported Belkaid was senior to Abdelhamid Abaaoud
, the team leader in Paris. Laachraoui's DNA was found at the bomb factory where the cell built the devices for the Paris attacks. The residence was a 15-minute walk away from the bomb factory.
The Belgian official told CNN that Laachraoui was possibly the ISIS cell's bomb-maker, but authorities have not recovered evidence definitively establishing this fact.
Investigators believe Laachraoui was the second suicide bomber in the Brussels Airport attack, multiple European officials told CNN.
According to the Belgian federal prosecutor, Belgian police found an ISIS flag, 15 kilograms of TATP, acetone, oxygenated water and screws at the Schaerbeek residence raided Tuesday. The information from the senior Belgian counterterrorism suggests that the attack the Brussels cell originally planned was going to be significantly larger than the one carried out Tuesday.
When a taxi driver picked up the airport attackers Tuesday in Schaerbeek, they had too many suitcases to fit in the cab, according to Belgian media outlet HLN. This report, together with the large amount of explosives recovered from the apartment, suggests the men had produced enough bombs for a larger attack team. The explosives were packed into suitcases each of the trio placed on a trolley when they arrived later Tuesday at the Brussels Airport.
The mayor of Zaventem, where the airport is located, told Belgian media that the men can be seen splitting up at the airport. Two detonated the device and one, pictured in closed-circuit TV footage in a light-colored jacket, ran away. It is not clear if his device did not work or he backed out of the plan.
He is being actively searched, according to Belgian counterterrorism officials.