CBI clinched the verdict by relying heavily on circumstantial

The CBI, which succeeded in its efforts today by ensuring conviction for Rajesh and Nupur Talwar in the murders of their teenage daughter and domestic help, relied heavily on circumstantial evidence and explored the conduct of the accused during the course of the probe.
There was no direct evidence against the dentist couple in the Aarushi-Hemraj double murder case, but CBI said in September, 2009 that all circumstantial evidences were against them which clearly indicate that only Talwar couple could have killed the duo.
As per CBI's theory, the circumstantial evidence are - no outside entry, last scene theory, dressing of crime scene, destruction of evidence, tampering with Aarushi's body and covering of Hemraj's body with cooler panel on the terrace - are against the dentist couple.

CBI officials said though there were all circumstantial evidences against Talwars, their suspicious behaviour, which has been from the beginning of this case, forced CBI's to frame the couple.
Talwars' maid Bharti Mandal's statement pointed finger on the dentist couple. Bharti said when she went to their house at around 6 AM on the morning of May 16, 2008, the outermost door was locked from inside.
"After ringing door bell several times, aunty (Nupur Talwar) opened inner latticed door and said to her through it that Hemraj may have gone to market for buying milk. Aunty threw the keys from balcony... When I returned and placed hands over the outermost door, it easily opened," Bharti said in her statement.

CBI's theory was that had any outsider committed murders, he would not have been able to lock door from inside.
As per CBI's claim, when police officials noticed blood spot on terrace door on the morning of May 16, 2008 and asked Rajesh to unlock it, he immediately asked police officials not to waste time in unlocking the door and search for Hemraj or he would escape.