Haldar was arrested from Raipur in Chhatisgarh in a joint operation by ATS and Crime Branch unit of city police earlier this month.
He said he is a "worshipper" of Asaram and did not take any money for killing the latter's personal doctor Amrut Prajapati in June 2014.
Haldar was also allegedly involved in the killings of Asaram's assistant-cum-cook Akhil Gupta in January last year and another key witness, Kripal Singh, in July.
"I came to know that Prajapti visits his Rajkot clinic every Friday. I came to Rajkot and fired at him when he was coming out of his clinic," he said.
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Haldar is currently being interrogated by Rajkot police in connection with Prajapati's murder.
"We have taken Haldar's custody from Ahmedabad (police) yesterday and would produce him before a local court," said a senior CID (crime) official.
Prajapati, the Ayurvedic practitioner, was shot at by two persons in Pedak Road area here when he was stepping out of his dispensary. He had later succumbed to his wounds while undergoing treatment at a Ahmedabad-based hospital.
Responding to a query, Haldar said he did not take any amount for gunning down Prajapti as he is a "worshipper of Asaram".
Police had said Haldar allegedly shot dead Gupta after the latter became a witness in the same case in 2013. He was fatally shot by the accused near his house in Muzaffarnagar, Uttar Pradesh. The probe in this case was handed over to CBI.
The sharp shooter then allegedly targeted Singh, who was killed in Shahjahanpur, UP. Singh used to work as a clerk in the office of a transporter whose minor daughter had lodged a case of sexual assault against Asaram in Jodhpur. Singh was the prime witness in that case, according to police.