A teenager was tied up and held at gunpoint Tuesday during a home invasion in Northborough, police said. Northborough police said the 17-year-old answered the door for two men whom he believed he recognized as repairmen who had previously worked at the Whitney Street residence. Once inside, one of the suspects tied up the victim and held him at gunpoint, while the other stole undisclosed items, police said. The teenager, who was unharmed, was able to free himself after the men left. Police said they have leads in the case.