Cashmere's brother said that they found the boy's sweater Monday morning.
Police are now calling the search a recovery mission. Rescue crews were on the scene all day Monday.
Nine hours into the search Monday, sonar equipment detected something promising not too far from where the 8-year-old fell into the river. But the water was so murky, the camera was not able to pick up anything. It took divers to get a closer look.
Specialized sonar equipment from the Wauconda Fire Department was brought in as well as a boat and other equipment from Lake and McHenry Counties.
Meantime, the Castillo family did not leave the scene all day.
Merrell Castillo spent the day near the bank of the river where Cashmere was last seen falling into the water Sunday night. He was playing tag when he hopped the fence and slipped.
"I hope my son is still alive," she said.
While the police and fire departments have been in recovery mode since Sunday night, the Castillo family is not giving up hope. On Monday morning, the boy's 13-year-old brother found a sweater that belonged to Cashmere near the river.
"I was just walking, saw a sweater. Picked it up and gave it to my mom and she was like, 'yeah, this is his.' But we are not sure if he was wearing his sweater or if he wasn't," said Christian Castillo.
"I keep searching for the whole night, running back and forth, working along both sides of the river, hoping that he is somewhere in there on those mud, sliding down, slipping or whatever," said Willie Castillo.
Marine units resumed their search at 7 a.m. Monday. Because the current is so strong and the visibility is zero, sonar equipment was used all day.
"It gives us some pretty clear images of what is under the water. We're starting here at this bridge, and we may end up moving further down the river to different points," said Deputy Chief John Escalante, Chicago Police.
Going from bridge to bridge, police said they will continue using the sonar equipment until the body is found or until it is dark.
While Cashmere's parents still have hope their son is alive, his brother is more realistic.
"If he passed away, our main aim is at least to find his body so he can then rest in peace," said Christian Castillo.
Good Samaritan, father jumped in after boy
Cashmere and his friends were playing along the North Branch of the river Sunday when he fell into the fast-moving current. His older brother was also nearby.
"We saw them by the river acting suspicious. I said, 'Why are they over there?' So, I just kept playing basketball, and then, I heard people just watching him saying, 'Hey, your brother is drowning.' And then I jumped. I dropped the ball, ran after him. I jumped. I had a split second to either jump in with him or just stay back," said Christian Castillo.
Christian Castillo said he yelled for help. A local resident came to the rescue.
"He was just walking with his family, and then I just told them, 'Can you please help me?' He jumped in. He took off his jacket and came swimming after him," said Christian Castillo.
The Castillo boys' father also tried jumping into the river.
"He jumped in for two seconds, but then I saw him coming back because the water was cold," said Christian Castillo.
Approximately two hours into the search, dive teams were pulled because the current was too strong and the visibility was zero.