May be they wanted to follow him to see where he was going or maybe they feared if they tried to take him in for questioning at home that they might provoke an incident putting other people in the home in danger. Was there a family or other people in the house - sorry what with dad being ill am only getting snippits of news so miss certain things.
I'm sure they locked down the house after he left...but they're looking for others...and they let him go as far as they could before he wiffed he was being followed.