One family who attended the service used to live near Millinocket, where the trooper resided with his wife, Hilary, and 6-month-old son Everett.
"It's a really tight community, really strong area for family and stuff like that," said James Nisbett, who now lives in Old Orchard Beach. "We wanted to come down just to support the family."
Law enforcement from across New England and as far away as New Mexico stood at attention to pay respects to their fallen brother.
The Patriot Riders also attended the funeral services.
Members say they remember Detective Campbell from a time when he helped escort them for the Summit Project.
"To realize that you know the person and he's not there, I'll ever be able to do that again," said Gregory Lozier of the Patriot Riders. "Those officers do that on their time. They take their day to escort us for a very good cause, for the Summit Project."
A public safety spokesman says 3,000 people attended the service.