In a Crime Watch Daily exclusive, investigators are revealing new information today about a high-profile unsolved murder, hoping the power of Crime Watch Daily and our viewers can finally crack the case.

It was just after noon on November 12, 2013. Chelsea Small, a mother of two, was working alone at a check-cashing store when a man in a hat asked to be buzzed into the store.

Gunned down on the job. A single mother's final moments captured on surveillance video. Was it a robbery that turned fatal from panic? Or had this killer directly come for Chelsea Small?

Four years after a mother's cold and calculated murder, the mystery deepens as detectives reveal an unusual new clue in the cold case.

Crime Watch Daily is teaming up with police to try to track down one of Michigan's most-wanted fugitives who could be living near you.

Everything 30-year-old Chelsea Small did, she did for her two kids.

"They were her life. Her babies were her life," said Chelsea's stepsister Mandy Stewart.

The single mother of two worked full-time at a cash-advance store in Taylor, Michigan just outside Detroit. She was also enrolled in classes at Wayne County Community College. Her sister Mandy says Chelsea was a standout student.

"She was on the dean's list," said Mandy. "She was very proud of herself. She studied very hard. Her children were her motivation. She was very motivated to give them and provide for them."

Despite her hectic schedule, Chelsea took time out to celebrate her son's fifth birthday, a day Chelsea's own mom Debi Kamin will never forget. Memories of that special day recently came flooding back when a lost camera suddenly resurfaced. Those family photos would be the last Chelsea Small would ever take.

Four days later Chelsea was on the job at Advance America Cash Advance, a day that began like any other.

"She was running to the bank to do the deposit when we talked briefly, and that was 10:30-ish, I guess, somewhere around that time," said Debi Kamin.

Ninety minutes later Chelsea Small was back behind the counter, alone in the store, which was kept locked for safety, even during business hours.

Shortly after 12 noon a man wearing a dark ball cap buzzed at the door.

"My personal gut as her mom and from day one I still believe that she knew him, or at least knew him through work," said Debi.

After a quick glance, Chelsea let him in.

"I think she knew of him enough or his face enough to not hesitate when she buzzed the buzzer to buzz him in," said Mandy Stewart.

Steven Schwein, the former lead investigator on the case who recently retired, agrees with the family.

"I think it's reasonable to think that she would be familiar with him, especially from a business standpoint," said retired Taylor Police Detective Steven Schwein.

Chelsea's mom says if the customer had raised any red flags, her daughter likely would have called before letting him inside.

"If she was afraid or intimidated, I would think she would've waited and called me like she had in the past, and said 'I just want to keep you on the line,'" said Debi.

But seconds after he entered the store, Chelsea Small was afraid. The man in the cap walked right up to the counter and pulled out a gun.

"The way he walks in there, he knew what he was after, he knew what he wanted to do," said WXYZ-TV Reporter Heather Catallo.

Just as Chelsea managed to hit the silent alarm, he fired the first shot.

"At that point, he's committed," said Schwein. "He fired a gun. He struck her. He knows he struck her. She fell to the ground. Now he's all in."

But he wasn't done yet. Surveillance video shows the cold-blooded killer walk around the counter and fire a second shot into the lifeless body of the mother of two.

"I think that his intent on going around to the back side of the counter is to make sure that Chelsea is not going to provide him any more resistance," said Schwein.

With the only witness now dead, the shooter begins searching the store, casually opening cabinets and drawers.

Taylor Police Detective Eric Jones, who now leads the investigation, says the killer's calm demeanor was astounding.

"There's no real panic. There's no real, 'Oh my gosh, I need to leave now,'" said Det. Jones.

After taking cash from the register the killer then strolls out the front door. His savage stop lasted just 60 seconds.

"It was incredible how calm and level-headed he was through the entire incident, especially when you consider what he did," said Steven Schwein. "There seemed to be no sense of urgency at any point."

If he'd come hoping to get rich, detectives say the killer likely left disappointed.

"It ended up being a small amount of money," said Det. Jones. "I would think he was expecting maybe more money."

Officers responding to the silent alarm arrived minutes later and found Chelsea's body behind the counter, but no trace of her killer.

Investigative reporter Heather Catallo from Crime Watch Daily Detroit affiliate WXYZ says the callous murder sent shockwaves through the community.

"The fact that this happened in such a populated area: Telegraph Road has like 60,000 vehicles on it every day," said Catallo. "And nobody saw anything."

Catallo believes the store's close proximity to Interstate 94 and the Detroit airport helped the killer make a quick escape.

"Even though it was busy, broad daylight, middle of the day, he had a ton of access to get out of there quickly," said Catallo.

The gunman was gone, but left behind is surveillance video of his cold and calculated crime.

"I've watched the video hundreds of times trying to see if something specific stands out," said Schwein.

And what stands out left them questioning if robbery was really the motive.

"I think the taking of the money is secondary," said Schwein.

Four years after she was murdered on the job, Chelsea Small's cold-blooded killer is still a free man.

"We've gone through hundreds, and I mean hundreds, of tips," said retired Taylor Police Detective Steven Schwein.

With the case now growing cold, detectives are revealing a critical piece of evidence to Crime Watch Daily they've kept secret until now.

"It's a key part of the case. It's a key part of what happened," said Det. Eric Jones.

The killer used a silencer during the shooting.

"Never once have I seen a suppressor used during the course of a robbery," said Schwein.

"I've never seen one in a case, and nobody around here has," said Det. Jones. "We've talked to a lot of people and it's very uncommon."

It's a clue cops hope will get a dangerous and callous killer off the street.

Do they think this guy is some professional hit-man, or do they think he just knows weapons and decides a silencer makes it easier?

"Maybe he's a hit-man, maybe he was there to do one thing and one thing only," said Det. Jones. "It's certainly a possibility, correct? I mean, I'm not going to rule out anything."

"Using an item like that suggests to me that she was an intended target and that was the number one goal, or somebody within the business was an intended target," said Schwein.

Whatever his motive for murder, Det. Jones says use of a silencer provides insight into the killer's state of mind.

"His state of mind is 'If I want to rob this place, I'm going to do whatever it takes to rob this place,'" said Det. Jones.

It may also offer critical clues into the type of person they're searching for.

"Based on what he has and what he uses, we know he probably owns several or is familiar with weapons," said Det. Jones. "Or he frequents gun ranges or works at one, or has been around one."

The killer didn't waste much time. Seconds after 30-year-old Chelsea Small buzzed him into the cash-advance store where she worked, the Michigan mother of two was dead, shot in the head and chest. The killer then briefly searches the Advance America store, stealing a small amount of cash from the register. Then he's gone, in and out in 60 seconds.

"Very brazen, very calm, very collected, no panic, no rush, none of that existed," said Det. Jones.

Retired detective Steven Schwein was the recent lead investigator on the case. He says the silencer has left him struggling to settle on a motive for the murder.

Does he feel the suspect came to kill her, or was it to rob that store?

"I'm usually pretty good at getting off the fence and picking one side or the other, and it's tough for me to stop straddling this fence," said Schwein.

But investigators say Chelsea Small had no known enemies. And nothing in her background indicated she was living in fear, or that someone would want to harm the mother of two, let alone kill her.

Given the picture presented on video, Schwein leans on the theory that the killer came into the store that day to commit a robbery.

"If that's his goal, to kill her, he goes around the back of the counter. There's a second shot," said Schwein. "At that point if his goal was to kill her, why not just leave? Why are you going to stay an extra six, seven, eight, nine seconds and rummage through the business?"

The retired detective believes the gunman opened fire because he knew Chelsea had triggered the silent alarm.

"My gut tells me that he stood there and in a matter of a split-second, he knew that she had triggered the alarm; she was a witness," said Schwein.

But until the killer is caught, Schwein says all options must be kept on the table.

"As investigators we have to be open-minded and consider every different scenario," said Schwein.

One thing they know for certain about the killer: Nobody is safe until they catch him.

"Somebody that can be that callous and somebody that can do that type of thing to anybody is a danger -- period," said Det. Jones.

Chelsea's family isn't giving up. Her sister Mandy is confident, and hoping Crime Watch Daily's national coverage will help lead to an arrest.

"I don't think he's in this area and that's why the right pair of eyes, somebody that knows him hasn't seen it yet," said Mandy Stewart.

"Justice for me gets him behind bars," said Chelsea's mother Debi Kamin.

And finally bring closure to a grieving family.

"It's been long enough," said Debi. "My grandkids and my family have suffered long enough."

Now maybe you can help bring justice to Chelsea's family. Police say the suspect is likely between 35 and 50 years old, standing 5 feet 10 inches tall with a stocky build, weighing about 200 pounds. He had dark bushy hair and was wearing a ball cap with an emblem on the front. Detectives believe his unique work-type clothing might trigger someone's memory.

"When we look at the video we see probably some sort of work boot along with some sort of work pants," said Taylor Police Det. Eric Jones. "On the one side of his pants you can see maybe it's like a white tag, like some sort of work pant, where if you worked at a business that issued work pants for people, employees, they have their tag number on it for dry-cleaning purposes or something like that."

Advance America is offering a $50,000 reward for information leading to Chelsea Small's killer. If you know who this man is you can call Crime Stoppers of Michigan anonymously at 1-800-SPEAK-UP.