Lal is suspected to have played a crucial role in the conspiracy for the blasts in October 2007 that left three persons dead and 20 injured, they said.
The National Investigation Agency (NIA) had on Tuesday arrested Bhavesh Patel from the national capital and produced him before a designated court in Jaipur. Patel was sent to 15 days custody.
Both the accused are likely to be interrogated about the conspiracy behind the bombing as well as the murder of RSS member Sunil Joshi, the sources said.
Patel is alleged to be one of the main persons involved in planting the explosives at a dargah in Ajmer, they said.
The NIA has arrested at least nine persons in connection with the murder of Joshi and explosions in Samjhauta train, Malegaon and Mecca Masjid blasts in Hyderabad.
The arrest of both the accused is expected to unravel the fate of other members of the terror module who have been absconding since the attack took place.