Hayes allegedly told Cowan that he didn't know if he could "go through" with killing Petit-Hawke, according to Krob, but when he spotted police cruisers outside the family's home that July day, he did it.
"He never mentioned how he killed Mrs. Petit. He just stated that he did kill Mrs. Petit," Krob testified.
Some of what Krob heard was in conversations between Hayes and other inmates using an inmate communication system.
Krob said Hayes talked with his cell neighbor by placing "empty toilet paper rolls and placing it over the sink drain and talking to each other through that system."
The officer told the court that Hayes had told the other inmate that investigators would find physical evidence of sodomy when they examined his alleged accomplice, Joshua Komisarjevsky. Komisarjevsky, will face a trial of his own at a later date.
"Hayes did mention that Komisarjevsky had taken cell phone pictures of the youngest Petit girl and was trying to e-mail them to his friends," said Krob.
Medical Examiner Wayne Carver has testified that the older of the two Petit girls, Hayley, was found laying face down in the hallway, but that the front of her clothing was more severely burned than the back, indicating she'd been directly exposed to fire.
He speculated that she had managed to free herself after being tied to her bed and made her way down the hallway and that Michaela had likely died a painful death.
The fire allegedly set by the two ex-cons was so ferocious that there was no chance of rescuing the three victims from the flames, according Paul Makuc of the state fire marshal's office.
The courtroom also heard testimony regarding a series of text messages that Hayes and Komisarjevsky sent back and forth before allegedly heading out around 3 a.m. to the Petit home, where Komisarjevsky had followed Hawke-Petit from a grocery store.
"I'm chomping at the bit to get started," Hayes wrote to Komisarjevsky at 7:45 p.m. the previous night. "Need a margarita soon."
And then, between 8:45 p.m. and 9:20 p.m., the following exchange:
"We still on?" Hayes sent to Komisarjevsky.
"Yes," Komisarjevsky replied.
"Soon?" Hayes wrote back.
"I'm putting kid to bed," Komisarjevsky then wrote to Hayes. "Hold your horses."
"Dude the horses want to get loose," Hayes replied. "Lol." ABC News' Sarah Netter and Lee Ferran contributed to this report.