How do they know
Sounds like a nonsense claim to me. I'm pretty certain that the spook responsible for India probably said immediately that this is probably Lashkar-e-Tayyiba as soon as they heard the reports and details. They probably even told their superiors their gut feeling. They then would have gone to work colloborating and finding evidence for what they thught they knew in the first few minutes of the attack before publically giving the terrorists publicity (last thing you want to do is give the credit to the wrong group and look silly)