Raghunandan Yandamuri, 26 is charged in the deaths of 10-month-old Saanvi Venna and 61-year-old Satayvahti Venna.
Investigators say Yandamuri went to the Venna family apartment armed with a knife back in October.
On videotaped confession, Yandamuri demonstrates how he held the baby in one arm and fended off her grandmother. He says he accidentally slashed her across the throat with a kitchen knife, dropped the baby, and then fell on top of her.
Yandamuri then describes how he tried to keep the crying baby quiet by stuffing a handkerchief in her mouth and securing it with a towel.
"I kept the kerchief in the mouth and I surrounded the head with a towel around the neck and tied it hard. But I don't know if that was too tight for her," he said on the video.
He then describes how he stuffed Saanvi into a suitcase and left her in an unused fitness room in the basement of one of the buildings at the Marquis Apartments on Route 202 in King of Prussia.
He says he went home and showered and then went to work.
When he arrived back home around 3:30 p.m. Yandamuri says he went to check on Saanvi and feed her, but she was already dead.
Yandamuri has a gambling problem, and investigators say he was hoping to ransom Saanvi for $50,000.
Police say Yandamuri joined the candlelight vigils and stood at the family's side as they pleaded for public help to find their daughter.