Two British nationals were detained by security agencies after they were believed to be indulging in suspicious activity.
The two — Stephen Hampston, who is in his mid-40s, and Steve Martin, 55, - were picked up from the Raddison Hotel near the Indira Gandhi International Airport on February 13, police said on Tuesday.
They were detained after police received a tip off that the two were carrying sophisticated equipment which could be used for air surveillance.
Police recovered high-tech binoculars from them.
Home Ministry sources said the movement of the Britishers have been restricted in the hotel itself.
Most of the agencies involved in the investigation have given clearance to the software found in the possession of the two British nationals, the sources said.