So, Monday morning in Billerica, a senior citizen (man in his late 60's, police told me), answered a knock on his front door a little before 8:30 a.m., and found himself face to face with a handgun and a stranger with a mask on his face.
The guy demanded money, got it and fled.
Nearby detail officers got to the scene within 30 seconds and helped secure the scene and keep the scent trail fresh.
Police used a K9 and tracked the suspect to a home nearby. An arrest was made within half an hour. Police say Matt Hill, 23, of 6 Kenilworth St., is charged with armed robbery while masked and home invasion. He is pictured here.

Police say the victim had no idea who Hill was. It was a random assault.
Police also used the Code Red system, which notifies homes in the area about the incident. It was just the second time they used it, I'm told. It's like reverse 911, except it can call way more people all at once since it is setup through a private contractor.
Go here for more information on Code Red.

Bridget Scrimenti did some video on this too.