3 Flee With $750,000 in Newhall Bank Heist : Robbery: The men threatened employees with a bag of what they said were explosives. They turned out to be flares and fireworks, police say.

Three robbers forced a Newhall bank manager to hold what they said was a bag of explosives and got away with about $750,000 Wednesday in one of the largest Southern California bank robberies in years, Los Angeles County sheriff’s deputies said.

After the robbers--described as three black men in their late 20s--escaped, deputies discovered that the bag actually contained road flares and fireworks taped together to look like dynamite.

Wednesday’s robbery occurred about 8:15 a.m., before the Bank of America branch on Valley Street was scheduled to open, Deputy Mark Bock said. Bock said that when the manager arrived for work, he was confronted by one of the robbers, who was holding a plastic grocery bag.

The robbers ordered employees to open the vault and empty money into two trash bags as the manager stood nearby with the bag containing the supposed explosives, investigators said. The employees were then led to the bank’s automated teller machines, where the manager was forced to lie on the ground while the workers removed canisters containing money.

The robbers tied up the manager and one woman employee and tried to lock them in the vault but could not close the door because of security bars that popped out when the door was opened, Bock said. The robbers then placed the grocery bag in the vault with the hostages and warned them, “Don’t come out or we’ll blow up the device,” Bock said.

After the robbers fled, an employee set off the bank’s alarm, alerting the Sheriff’s Department. Learning that the bag was still in the vault, deputies closed off about one block of Valley Street and evacuated 18 residences until it could be determined that the bag’s contents were harmless, Bock said.

The exact sum taken was not determined, but it appeared to be about $750,000, according to sheriff’s Lt. Barbara Persten. FBI officials declined to reveal the amount.

The loot apparently exceeds the $430,000 taken by robbers at a Tarzana bank in 1992, which at that time was the largest bank robbery in Los Angeles history. Whether more recent robberies have exceeded that figure is difficult to determine, because the FBI--which has jurisdiction over investigation of virtually all bank holdups--and bank executives are reluctant to reveal robbery amounts for fear of encouraging the crime. “We’re not going to advertise how lucrative some bank robberies can be,” said John Hoos, an FBI spokesman.