WATERBURY, Conn.--At least two men broke into a Purolator Security Inc. office shortly after 4 a.m. yesterday, killing three guards and escaping with $1.79 million in cash, checks and jewelry, police said.

The thieves also stole $40 to $50 million in negotiable securities, an investigator said.

Sources report the thieves, armed with high-powered rifles, waited behind the Purolator garage and fired through a garage door after the arrival of an armored truck carrying money from Hartford for delivery to several New York City banks.

Westbury Chief Inspector John T. Griffin declined to disclose whether there are any suspects and whether the robbery appeared well-planned.

"The perpetrators were waiting in ambush for the people inside the armored car." Griffin said in a news conference yesterday.

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Purolator President John Pratt said yesterday the robbery is the largest in his company's history. $4.3 million in cash was stolen in Chicago from the company in 1974, but all but $900.000 was recovered. Pratt said. Purolator is one of the nation's largest armored car manufacturers.

A Purolator employee discovered the bodies of the three slain guards at 7:15 a.m. yesterday in the garage area.

The men killed were William West, age 52; Leslie Clark, age 46; and Hartford policeman Edward T. Cody.