What starts with a simple knock on the door can quickly turn into a violent robbery.

Some criminals are posing as utility workers or delivery drivers to break into residences, tricking unsuspecting people into opening their doors and allowing the thieves into their homes.

To see if homeowners are vulnerable to this type of crime, Inside Edition outfitted former law enforcement officer and security consultant Steve Kardian with a hard hat, vest and clipboard, and sent him to knock on doors around Westchester County, New York.

Kardian told homeowners he was with the water commission and asked whether he could check their water pressure. But what many didn't realize is there's no such agency as the water commission in that neighborhood.

Chief Investigative Correspondent Lisa Guerrero watched from a car out front along with Officer Jonathan Kramel with the Mount Pleasant Police Department.

"I'm with the water commission ... we're checking on the water pressure. Do you mind if I come in and check?" Kardian asked one woman.

"OK," she replied.

"You sure?" he checked.

"OK," she said.

After she agreed, the team explained the situation. "I'm actually with Inside Edition," Kardian said. "... There's been dozens and dozens of cases of people posing as utility workers getting into the homes and doing horrible things."

"So were you convinced?" Guerrero asked the woman.

"I was ... it looked like a pretty convincing utility person coming to check on the water pressure," she replied.

"So were you going to let him into your home?" Guerrero asked the homeowner.

"I was," she said.

Another woman also opened the door without thinking twice.

"So you actually thought that he looked convincing?" Guerrero asked the second woman.

"Oh absolutely," she answered. "Most people are trusting."

"That's why we're doing the story," Guerrero explained.

"You caught me in this one," she said. "I'm glad you did. I will be much more careful."

Not everyone was so trusting.

One homeowner asked Kardian for identification when he knocked on his door.

"Listen, show me a card and you're in," he said.

"Congrats, you did the right thing," Guerrero replied.

So next time someone knocks on your door, make sure you know who you're dealing with before you open it.

"Ask for identification," said Kardian. "And if you get that feeling that something's not right, make that phone call. Call the police. Call the company, but don't let him in your house!"

Utilities United Against Scams warns you should never let anyone into your home unless you have a scheduled appointment or you have reported something wrong. The trade association group also warns of scammers calling or emailing you with threats of shutting off your utilities unless you send payment.

For more information and tips on how to protect yourself you can visit here.

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