Terrifying calls, secret surveillance and deadly threats -- all the work of cell phone hackers who can literally take control of mobile phones and use them to stalk and harass at will.

In a special report, KIRO 7 Eyewitness News South Sound reporter Kevin McCarty takes an in-depth look at how the hackers work -- and how you can protect yourself and your family.

"I will kill you," said one of the messages.

For months, the calls have come with a strange voice uttering threats.

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It was a cell phone hacker -- using technology to terrorize.

Whoever makes the calls knows intimate details of the people he is targeting -- even threatening the schools their children attend.

"There will be a shooting at Curtis and Whittier today. So don't send your kids. They will be dead, so dead," said another message.

What's more frightening for Heather Kuykendall of Fircrest -- whose 16-year-old daughter Courtney has been the focus of many of the calls -- is that the hacker has somehow taken control of three family cell phones using them as monitoring devices to track her movements.

"Sometimes, she doesn't like to tell me everything that they say. She knows that it just really upsets me. But from what she told me they just knew where she was specifically and they let her know that there were there, too," Heather Kuykendall said.

"I know where you are, I know where you live."

But how is all this possible?

KIRO 7 has been looking into the technology hackers use -- allowing them to turn cell phones on and off, use mouthpieces as microphones, watch and even photograph unsuspecting targets through their cameras -- all remotely.

According to the cell phone companies, that can't be done.

Sprint -- the cell phone provider for the families involved issued this statement:

"We are unaware of technology that would enable the activity portrayed in this story to occur."

"What they're describing is completely credible and is technically quite possible," said James Atkinson of the Granite Island Group near Boston.

Atkinson has specialized in counter surveillance and anti-bugging technology professionally for more than 20 years.

He says all the hackers need is the right software.

"Gaining the software is just a matter of clicking on several hundred Web sites that have the software free of charge, the instructions are readily available free of charge," he said.

"They left a message saying, 'You tell Kayleen we're coming to get her,'" said Kayleen Zimmerman.

Zimmerman, 15, has also been targeted by a hacker -- possibly the same individual -- who stalks, taunts and threatens her over and over.

"I don't know. I think it's some teenage boy who got turned down from girls and wants to get back or something like that, but I don't know. I don't know who it is. It could be anybody," she said.

"It's kind of a cat-and-mouse game, where the hackers are always looking for new ways to get into it," said Dr. Michael Foley of Bluetooth SIG.

Cell phones have another gateway open to hackers -- Bluetooth.

Wireless connectivity to headsets and PC's means information is in the air -- and the hackers are listening.

"Now the bell's kind of been rung and we're not going to be able to unring it and that's something that in the future, we're always going to have to be aware of our surroundings and how to best keep our devices secure."

There are ways you can keep a hacker out of your phone. Don't accept messages, pictures or emails from people you don't know. If you're not using Bluetooth, turn it off.

You can also buy software that can block the hackers, until they figure out a way around that.