A police officer today said that the accused had visited the victim's house earlier and knew that she resided there alone.
The accused, identified as Haider, was arrested last night from Mandavali area for alleged robbery and murder of Sumanbala Bhatnagar (70) in Chitra Vihar area.
Preliminary investigation revealed that the suspect had gained a friendly entry in the house to commit robbery, following which the police analysed the CCTV footage.
In the footage, Haider was seen leaving the area at that time, said Ajay Kumar, DCP (East).
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Police have recovered a hammer, a motorcycle and Rs 1,100 in cash from his possession.
During interrogation, the accused said due to his alcohol addiction, he was facing financial hardship and decided to rob the woman's house, police said.
As she tried to raise an alarm, the accused repeatedly hit her on the head and smothered her with a pillow, police said, adding that he robbed her of her earrings and cash.