As a light rain fell, they cut a hole in the roof of the 70,000-square-foot building and slid down ropes to get inside.
They disabled the building's alarm system, and police believe they loaded several dozen pallets of prescription drugs — worth $75 million — into at least one truck parked at the rear of the building, and drove away.
The heist wasn't discovered until the afternoon, when an employee went to work.
The theft at the Eli Lilly & Co. warehouse, one of three distribution centers in the nation for the international pharmaceutical firm, was well-planned and "extremely substantial," Police Chief Carl Sferrazza said.
Edward Sagebiel, a spokesman for Eli Lilly, said Tuesday that the theft "certainly has the appearance of a sophisticated, well-planned criminal action."
"We are conducting a full investigation and working with authorities ... with the intention of recovering the products," Sagebiel said.
The burglars were sophisticated, the police chief said. He said he didn't know what type of drugs or how many pallets were taken, but it was enough to fill at least one tractor-trailer.
"I can tell you it was many, many pallets," he said Tuesday morning. "They might have spent at least a couple of hours unloading all these drugs."
Sferrazza said that police were reviewing surveillance cameras and interviewing employees at the facility to gather evidence. Detectives will look for common crimes in other parts of the country or the state.
Two Enfield detectives have been assigned to the case full time, and the FBI will be meeting with Enfield police today.
"We're putting a lot of time and a lot of effort into this," Sferrazza said. "The burglary is unique from a lot of perspectives."
"This will turn out to be, unfortunately, the largest theft that our town has ever experienced," he said.
Sagebiel said that the drugs stored at the facility include Prozac, Cymbalta and Zyprexa, but that he could not say which drugs were taken. No narcotics or other painkillers were stored at the warehouse, he said.
He described the take as "several dozen pallets of products that range across our portfolio."
"It's hard for us to speculate on the intent, but we know that there is a marketplace out there on pharmaceuticals," he said.
Police were dispatched to the warehouse Sunday about 1:50 p.m., when the theft was discovered, according to a police report. A state police dog was called in to search for suspects, but none was found.
Sagebiel said that corporate security officials were at the site, and that any necessary security improvements would be made.
The Enfield site is Eli Lilly's East Coast distribution facility. The company is based in Indianapolis.