Authorities in Fort Bend County in Texas said three suspects tortured a 7-year-old boy to force his father to give them money. Picture: Fort Bend County Sheriff's Office.

Richmond, Texas - For 40 minutes, authorities say, three masked suspects held a father, a mother and their two small children hostage.

They demanded money that the father said he didn't have, and then they beat him for not giving them what they wanted.

Even more disturbing, authorities in the Houston suburb say, the home invaders turned on one of the children, a 7-year-old boy, torturing him in a bathtub filled with hot water.

The chilling screams from family members can be heard in a recording made inside the home in Richmond, Texas, on Monday morning.

"You can hear the boy screaming, and then you can hear them when they're filling up that bathtub with water," Fort Bend County Sheriff Troy Nehls told reporters at a news conference.

Then, the boy's screams turned into muffled sounds.

The suspects used the boy as leverage to force the father to tell them where he kept his money, authorities said. They took him to the tub they had filled with hot water and dunked the boy in it, repeatedly.

"What amount of money would it take for any of us to torture a 7-year-old kid? No amount of money," Fort Bend County Sheriff's Detective Justin White said at a Tuesday news conference. "But, you know, here they are doing it. For what? A couple of thousand dollars they think they may have at the house or something like that? To me, that's just evil."

As of Wednesday, the suspects have not been caught.

Investigators say the suspects had their sights on the family's home in Richmond, southwest of Houston, but could only speculate about why they targeted the victims.

The father owns a small business in the area, authorities told reporters, and the home invaders seemed to be familiar with the house.

"For whatever reason that we don't know right now, they believed that there was a large amount of money inside that house," White said, declining to share more details.

Nehls, the sheriff, said the family was awakened by a loud noise about 3 a.m. Monday. Surveillance video taken from outside the house showed the three suspects just seconds before they kicked in the door. They pistol-whipped the father as they demanded money, leaving a bloody gash on his head.

"And the father kept saying, 'There is no money. There is no money here. You could take my jewellery. You could take other things. Take the car, but we have no money,'" Nehls said, describing the audio.

Nehls said that "didn't sit well" with the suspects, so they turned on the man's oldest son, the 7-year-old.

After holding the boy underwater, one of the suspects walked him around the living room and kitchen area to try to find the money, authorities said.

"It's just horrible to think that we have three individuals out there that are trying to make a little extra money and they're going to do that to a young man like that, a child," Nehls said. "How dare you."

Sheriff's department officials said the audio and surveillance video from inside the house documented the 40-minute-long horror.

It's unclear how the audio was recorded, and officials said they aren't releasing the recordings, citing the pending investigation. But still photos released by the department showed one of the suspects with his hand on the barefoot boy's shoulder as he walked him to the living room.

The home invaders can be seen walking around the house, searching. One of them, authorities said, stood on the counter to look inside the cabinets.

One of the suspects, who was seen holding a gun, was wearing a vintage Jordan jacket with white sleeves. Another was wearing an Adidas pullover with a white shirt that had the word "CHANGE" printed in front. The third suspect was wearing red sneakers.

Authorities said the group left with the family's cellphones. The victims went to an urgent care center, where 911 was called.

Nehls said the boy had fluid in his lungs but did not suffer burns from the water. He and his father have been released from the hospital. The mother and the other child, a 5-year-old, were not harmed.

"I know adults have been beaten before, but I've never seen in my career anyone ever try to use a boy to try to get information from the father," Nehls said.

Authorities have offered $25 000 to anyone who can help them catch the three suspects along with a fourth person who was their getaway driver.

"They didn't get what they wanted in this one. So when are they going to try another one?" White said. "And whose family is going to be next? Whose child is going to be next?"

The Washington Post