The National Investigation Agency (NIA), which is in touch with the Punjab Police, may start probing the Mohali attack case, sources said. A team of NIA's terror unit is in contact with the Punjab Police. The NIA believes that Khalistani groups are thriving in Punjab who have carried out the attack after conducting reconnaissance of the area. A rocket-propelled grenade (RPG) was fired from outside the street, which landed on the third floor of the Punjab Police's Intelligence Headquarters in sector 77 of Mohali. The grenade, however, did not explode and only the glass doors and windowpanes were shattered. It is said that RPG-22 might have been used in the attack. Four days ago four Khalistani terrorists were arrested by the Haryana Police. The preliminary probe has indicated that two car-borne persons might have been involved in the attack. A swift dzire car is said to be seen outside the Intelligence wing before the attack took place. As the building doesn't have any CCTV camera, the police are trying to gather footage from the nearby area. A number of persons living in the neighbourhood have been questioned. The NIA is also gathering information about the incident. A team from the Delhi's office might be sent to Mohali after the instructions from the Union Ministry of Home Affairs. "A terror unit of NIA is in touch with the Punjab Police. A few Intelligence reports were issued which said the Khalistani outfits were active and they were planning to disturb peace in the area," said a source.