LOS ANGELES (AP) _ It has been a deadly year for Southern California money messengers - 13 armored car holdups, three possibly related bank robberies, two slayings and the theft of hundreds of thousands of dollars.

The fatal shooting of an armored car guard during a stop at a supermarket Dec. 15 was the latest in the rash of heists that have often occurred during busy daytime hours and frequently ended in gunfire.

''The only time you feel safe is when you're in that truck and it's for very few minutes until your next stop,'' said Nick Scarpelli, a guard who quit after a friend at Armored Transport of California was slain in October.

The crimes began Dec. 18, 1987, and have involved 13 armored cars, a bank and two savings and loans in Los Angeles, Riverside and Ventura counties. Three police departments, two sheriff's departments and the FBI are investigating.

The amount stolen in every robbery has not been revealed, but Los Angeles police Detective Norman Roberge said the total is in the hundreds of thousands of dollars. In one theft, two gunmen held up a guard leaving a Universal City theater and took $166,000.

''There have been sporadic robberies in recent years past but none where it's as organized as this; I mean a rash of armored car robberies in what appears to be a small nucleus of individuals,'' Roberge said Wednesday.

''We're looking at them with possibly the same suspects being involved in some of these robberies,'' he said. ''We're also looking at avenues where some of these robberies may have been committed by some imitators.''

Two men have been arrested for investigation of a June robbery at a supermarket in Los Angeles, and 29-year-old Steven Bernard Young was arrested and booked Friday for investigation of robbery in the armored car probe, police said. A brief police statement issued Sunday said Young was ''one of the main suspects'' in some of the heists but gave no details.

The overall probe continues into what have been unusually well-planned and violent crimes in which the gunmen have been more willing than usual to shoot and have not been deterred by the presence of bystanders, police said.

Both slain guards worked for Armored Transport of California Inc., which has been struck seven times.

Stewart Cruz Tecson, 27, was killed during a stop at a bank in the San Fernando Valley on Oct. 14. Patrick Rooney, 35, was killed at midmorning as he picked up cash at a supermarket in suburban Bellflower on Dec. 15.

''All I can say is it was a cold and ruthless, execution-type killing,'' Roberge said. ''It was uncalled for.''

Joel Curnutt, director of operations for Armored Transport of California, said the company had no comment while the robberies were under investigation.

Scarpelli said he quit two days after Tecson was slain, and he said about 20 other guards have also either quit or given notice.

''My family couldn't take it anymore,'' said Scarpelli. He said each armored car should have a crew of three so that the one carrying money bags can be covered by an unencumbered guard, leaving a third man to stay in the truck.