The navy says Lazcano was killed after marines tried to search a group of suspicious men outside a baseball stadium, after receiving a tip there were armed men in the area. The men fired on the marines and a firefight raged, it says.
Rear Adm. Jose Luis Vergara said in radio and television interviews Wednesday that Lazcano's body was left at a funeral home along with the body of an accomplice after the gunfight because marines believed he was just a common criminal and didn't suspect they had just taken down the leader of the Zetas cartel.
"For us it was two more criminals. We had no indication that it was 'Lazca,'" the admiral told MVS Radio.
Vergara said authorities only realized they had killed a significant figure when armed men stole the body from the funeral home. Checks of fingerprints taken from the body confirmed the dead man was Lazcano.
The spokesman also revealed for the first time that a third suspect was present during the gunbattle and escaped.
He said it was increasingly common for drug-cartel leaders to travel in small groups instead of heavily armed convoys, in order to attract less attention.
The fallen capo was an army special forces deserter whose brutality and paramilitary tactics transformed a small group of drug cartel enforcers into one of the world's most feared international criminal organizations.
Analysts say his death could set off a power struggle inside the Zetas as its relatively autonomous local cells decide whether to align with its remaining boss, Miguel Angel Trevino Morales, a man considered even more ruthless and brutal than Lazcano.