Haroon was arrested after the Gujarat ATS sent specific intelligence input, said media reports.
Meanwhile, the Crime Branch officers have claimed to uncover the details of venture between the terror outfits-Indian Mujahideen and Taliban.
"Emails and correspondence of Danish with five top terror operatives, including Haroon of Kolkata, have revealed this conspiracy across international borders. Haroon had also been to Hyderabad to meet Danish to discuss their plans," said a senior official to The Times of India.
Danish Riyaz, a key IM operative was arrested by the Crime Branch in Jun.
According to reports, the alliance was formed before Danish was arrested. The Times of India quoted another senior official in the crime branch as saying, "According to Haroon's emails, 10 hand-picked youth from across the country were to be sent to Pakistan and then Afghanistan for terror training."
The police also claimed that Haroon is a suspect in the 13 Jun Mumbai blasts and is said to have been in contact with Talibani leaders. Police are also investigating the matter if 10 youth were sent for special Taliban training program.