This comes after Mandal’s polygraph (lie-detector) test result showed “deception” in his statements, CBI sources said.

Investigators initially probed the possibility of the presence of more than two persons in the Talwar residence on May 15 night, when Arushi and household help were murdered. CBI has already arrested the dentist couple’s compounder, Krishna, and family friend Anita Durani’s help Rajkumar following “adverse” reports in their lie-detector tests.

But now, with Mandal’s lie-detector test report also hinting at the fact that he might be lying, investigators believe he might be hiding facts. Mandal, 24, underwent a polygraph test on June 19, three days after the probe agency picked him up after Krishna reportedly took his name during interrogation.

Mandal is still being interrogated, though CBI has not arrested him yet.

Sources said he has lied about several facts, including his presence in the Talwar house on the night of the murders, and that he changed his statement to say he did not know Krishna and Rajkumar. This, sources said, despite the investigators already having it established that he was a “good friend” of the duo and often met them.

The investigators have already taken Mandal to the crime spot — L-32, Jalvayu Vihar, Sector-25 — several times and he is being repeatedly questioned at the CBI headquarters on Lodhi Road.

Mandal is employed with one Puneet Rai Tandon, a resident of the same block in Jalvayu Vihar.