SANTA ANA, Calif. » Three California inmates who sawed through a metal grate, crawled through plumbing tunnels and shimmied to freedom down a rope made from bed linens likely had help to pull off the daring plan and also benefited from the complacency of jail staff, security experts said today.

The inmates vanished early Friday in a jail break eerily similar to the escape of two inmates from an upstate New York prison last summer. Those men also cut through a portion of wall hidden under a bunk bed and used piping and tunnels inside the facility to reach the outside.

The California inmates, including one who is charged with murder, were still at large Monday. Jail officials did not realize they were missing until roughly 16 hours after they were last seen because an evening headcount was delayed by an assault on a guard.

A major question for investigators will be how the men were able to plan and execute their flight with such precision, said Kevin Tamez, a managing partner for MPM Group, a Philadelphia-based firm that consults on prison security, management and infrastructure.

It’s likely someone slipped them blueprints or told them how the bowels of the jail were laid out, he said.

“If I were whoever’s investigating, there are some people who would be on a polygraph, I guarantee you,” Tamez said. “They had to have had some inside help.”

Lt. Jeff Hallock, a spokesman for the Orange County Sheriff’s Department, said there is no evidence so far that the trio had help from the inside but authorities know it’s a possibility.

Jonathan Tieu, 20, Bac Duong, 43, and Hossein Nayeri, 37, were all awaiting trials for unrelated violent crimes at the Men’s Central Jail. They were held in a dormitory with about 65 other men.

They cut through a quarter-inch-thick grill on a dormitory wall and got into plumbing tunnels before sawing through half-inch-thick steel bars as they made their way behind walls to an unguarded area of a roof atop a five-story building. There, they moved aside razor wire and rappelled to the ground using the bed linen.

It was the first escape in nearly three decades from the California facility built in 1968. The jail holds 900 men and is located in Santa Ana, about 30 miles southeast of Los Angeles.

The similarities to last summer’s escape in New York point to complacency among guards and administrators at the California facility, experts said.

“This summer we had this huge escape from Clinton Prison in New York and every prison or jail administrator in the country should have said to themselves, ‘Huh, I wonder if I am vulnerable?’ and should have checked their steam shafts and tunnels and every other thing that gives access to the outside,” said Martin Horn, a professor of corrections at John Jay College of Criminal Justice at City University of New York.

Motion sensor cameras — available for $55 and often used as baby monitors — can be installed along interior tunnels and pipes to catch inmates, Tamez said.

Thorough searches of dorms likely would have discovered the tools used or damage to the vent grill, he said.

Hallock said the jail’s general policy is to do walk-throughs every hour to check on inmates. More involved searches are done randomly, he said, but declined to be more specific.

It’s also unclear why the inmates — who were charged with violent felonies — were housed in a common dorm with more than 60 other inmates. Assignment to a large, busy room likely made it easier for them to avoid detection, Horn said.

Authorities believe Tieu and Duong remain in Southern California, possibly hiding out in the local Vietnamese community in Orange County. Officials held a news conference in Vietnamese to ask for help.

Federal authorities are offering $50,000 in rewards for information leading to their recapture.

Tieu had been held on a $1 million bond since October 2013 on charges of murder, attempted murder and shooting at an inhabited dwelling in an alleged gang dispute.

Nayeri had been held without bond since September 2014 on charges of kidnapping, torture, aggravated mayhem and burglary. Authorities say he kidnapped a marijuana dealer, burned him with a blow torch and cut off his penis because Nayeri thought the man had buried money in the desert.

Duong has been held without bond since last month on charges of attempted murder, assault with a deadly weapon and other charges in the shooting of a man on his front porch.

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Associated Press Writer Amy Taxin contributed to this report.