Suspected Lashkar-e-Tayiba terrorist Fahim Ansari is the key to cracking the Mumbai terror attack case, investigators believe.

Ansari who was arrested in connection with the Fidayeen attack on the Central Reserve Police Force camp at Rampur on January 1, has given out a lot of details regarding the Mumbai terror plot.

Uttar Pradesh [Images] police, who have been questioning him once again since the November 26-29 attack, say the man was aware of the attack and had played a major role in plotting it.

The police say he will be the key to ascertaining whether there was Indian participation.

During the latest round of interrogation, the UP police have found that Ansari had touched base with Tausif Rehman and Mukthar Ahmed Sheikh and had asked them to get SIM cards. The brief was for the two men to buy the SIM cards and pass it on to the men who finally carried out the Mumbai terror attack.

The two were asked to get 20 SIM cards, which they finally did by producing fake documents.

Further, Ansari said during his interrogation that he helped operatives of the Lashkar gather video footage of the targets in Mumbai and also helped in gathering route information for the terrorists.

This gives an indication that Ansari had slipped into India in 2007 and had reconnoitered Mumbai before he went back to UP and carried out the Rampur attack.

Intelligence Bureau sources told rediff.com that the Rampur attack could have been a precursor to the Mumbai attack. The Lashkar had planned on carrying out the Mumbai attack in September itself and the Rampur attack was meant to act as a distraction for the bigger attack.

However, there was a change in plan following the arrest of Ansari and others. With Ansari spilling out the details of the attack, the Lashkar-e-Tayiba was forced to postpone the Mumbai attack.

All the 24 men who had trained for the Mumbai attack were asked to stay back in Pakistan and leave at a later date.

Ansari was arrested in February. During the first round of interrogation, he had revealed specifically that the Lashkar had planned on targeting Mumbai.

However, none of the investigating agencies had taken him seriously and chose to ignore the open warning that he had sounded. The map which showed details of the Mumbai attack was also buried under the charge sheet with none taking it seriously.

The IB says it had sounded off a warning based on the intelligence received by them and also on the statements made by Ansari. However the warnings were ignored stating that the intelligence was not actionable.

The Mumbai police, who have sought Ansari's custody, are however non-committal on why they ignored the inputs based on Ansari's statements.