But unbeknownst to the guard, he had inadvertently unlocked the door that opens to the outside.
"That may sound crazy, but these people are crazy like a fox," Underwood said.
Underwood said the inmates' plan was "well laid out," and they took advantage of an inexperienced employee.
"This is one time we slipped up. I'm not going to make excuses. It was a human error that caused this to happen... He's a young guy, hadn't been here that long," Underwood said of the jail guard whom he did not name.
The 12 inmates then used blankets to cover and climb over a razor-wire fence that surrounds the Walker County Jail in Jasper, Alabama, northwest of Birmingham.
Several of them rode in one truck, while the others ran in different directions, Underwood said.
Eleven have since been recaptured and are now in custody. One, Brady Andrew Kilpatrick, who was in jail on drug possession charges, remains at large.
The inmates are between the ages of 18 and 30. Two are in jail on attempted murder charges. The others are charged with crimes such as burglary, theft, drug charges, domestic violence, breaking and entering, and disorderly conduct.
Underwood said the inmates will face additional escape charges. Others who helped them after they escaped will also be charged. He did not say what will happen to the jail guard, only telling reporters, "We're going to take care of that matter."
Underwood said this is the first time inmates escaped while he is in charge of the jail.
"It happens. Escapes happen," he told reporters, citing two other counties that had the same problem.
"When you deal with people because you locked them up, they're still criminals. They're going to continue what they're going. They don't become nice just because you got them locked in jail."
He added: "So we got some evil people down here, and they scheme all the time to con us and our employees here at the jail. You have to stay on your toes."
The nearly 20-year-old facility now has 240 inmates, Underwood said.
Underwood also acknowledged that the jail needs some improvement, such as installing more monitors and cameras. He said officials will look into whether a new or young employee should be assigned to the control room.
What is likely the most famous prison escape in recent years happened in 2015, when two convicted murderers used power tools to drill through steel walls and pipes inside a maximum-security prison in upstate New York.
Hundreds of law enforcement officers swarmed neighborhood streets in a manhunt that lasted for three weeks.
A prison worker was charged with helping the men escape by smuggling power tools into the facility.
In one recent jailbreak in Oklahoma, two inmates attempted to break out by climbing through the jail's ventilation system. They were caught but were able to flee again through the vent system three months later, when they were joined by two more inmates.
Last December, six Tennessee inmates escaped by ripping a stainless steel toilet mounted to a wall. They were all arrested a few days later.
And in Southern California in January 2016, three inmates cut through steel bars in the facility's dormitory and went through plumbing tunnels to reach the roof. They then used blankets as a rope to get down to the ground.
One of the three - the last to be captured - was found 640km away in San Francisco more than a week later.
