Saching Tyagi, 27, an Uttarakhand Police constable and Namdhari's PSO, was arrested in New Delhi on Monday for his alleged role in the conspiracy that led to the murders of Ponty and Hardeep Chadha, said SBS Tyagi, Deputy Commissioner of Police (Crime).
Mr Tyagi has been charged with criminal conspiracy and other sections, said the officer.
Earlier, the police had arrested Mr Namdhari on charges of allegedly murdering Hardeep while six others were arrested on charges of trespassing.
During investigations, Sachin Tyagi told the police that he had fired at Hardeep with his official weapon.
Sachin Tyagi joined Uttarakhand Police as a constable in 2005 and was attached with Mr Namdhari as his PSO since January 2012.
Asked about Sachin Tyagi's role, the officer said that he allegedly gave false statements to investigators and helped destroy evidence.
"Namdhari was in possession of a .30 caliber pistol but he did not have a licence for bringing the weapon to Delhi," said the officer.
He claimed that Mr Namdhari gave the pistol to Sachin Tyagi.
"Sachin Tyagi handed over the firearm to one Sonu who in turn gave it to Satnam. Sachin Tyagi knew this was an important evidence but he helped Namdhari," said the officer.
Special teams of Delhi Police have been sent to Uttarakhand and Uttar Pradesh and two more people are likely to be arrested soon.
"Two more people related to the Chadha brothers' murder case would be under police custody till late Monday night," said SBK Singh, Joint Commissioner of Police (Crime).
According to Mr Singh, the people are related to Mr Namdhari and they have a major role in trespassing at south Delhi's Chhattarpur farmhouse No. 42 where the Chadha brothers were killed in a shootout on November 17 over the property's possession.
However, the police are still working on the theory whether Mr Namdhari shot both the brothers or he fired only at Hardeep.
"We are investigating each and every angle and Mr Namdhari's role is suspicious in the gun-battle. Our investigation is still on," SBS Tyagi said.
The police were waiting for the ballistic report which would confirm the type of weapons and bullets used in the shootout.