TEMPE, Ariz. -- The FBI has doubled the reward for information leading to the arrest of three men who took two families hostage in a well-bred plot to steal at least $400,000 from a Tempe bank.

The reward was upped Monday to $100,000.

Video: FBI Doubles Bank Robbery Reward

The heist was one of the largest bank robberies of its kind in Arizona history, said John Lewis, Phoenix FBI Special Agent in Charge.

And Lewis said the robbers are still on the loose.

"We anticipate after they blow through this $400,000 they will do it again," Lewis said.

"It's worrisome for us because we have individuals who are armed to the teeth with multiple weapons," Lewis said.

The men, described as young with athletic builds, pulled off the robbery Friday morning at a Wells Fargo bank at 8670 S. McClintock Dr.

FBI: Robbers Clever, Brazen

The night before around 7:30, they entered the home of a Chandler couple, one of whom is the bank manager, Lewis said.

They handcuffed the couple and held them hostage until 4 a.m., according to Lewis.

"The robbers had some concern for the well-being of their victims," Lewis said.

"Although at one point they threatened them that if things went wrong, they would do what they needed to do," Lewis said.

Around 4 a.m., the men drove the manager to the bank where they waited until 8 a.m. and took several arriving bank employees hostage, Lewis said. The robbers were brandishing weapons.

"One of the weapons involved is described as a semi-automatic," according to Lewis.

They ordered the manager to open the safe, cleaned out the vault and fled, according to Lewis.

No one was harmed.

Lewis said investigators think the men were driving a light or silver-colored 2001-2008 Plymouth or Chrysler minivan with the rear seats removed.

Earlier Hostage Situation Failed

Besides the amount of money taken, Lewis said this case if different from average bank robberies.

"Because typically we have some pretty decent leads to follow up on," Lewis said.

"But that's not the case here. We do have some information we're following," he said.

On Wednesday night, the men had gained entry into another home where they held a couple and their young child hostage for about an hour, Lewis said.

The FBI said it believes the men were going to carry out the same plan to rob a Desert Schools Federal Credit Union in Chandler.

But they aborted the scheme when they apparently realized they weren't going to gain entry to the facility, Lewis said.

One man is described as between 25 to 35 years old, 5 feet 9 inches to 6 feet tall with a slender build. He has brown eyes and dark hair on his forearms.

A second man is described as 5 feet 9 inches tall and weighs between 190 to 210 pounds. He also is between 25 to 35 years old. The FBI said it believes he is a smoker because his hostages said he was hacking or coughing.

"Two individuals wore all-black tactical-style clothing, while another subject posed as a construction worker wearing a mustard-yellow construction hard-hat and reflective vest," Lewis said.

"One of these subjects we know had on his left hand a wedding band," Lewis added.

"It was a dull gold finish, not ornamental," he said.

Gunmen Conducted Keen Surveillance, Agent Says

Lewis said the men undertook extraordinary amounts of surveillance.

"From the banks to the homes. Very difficult to pull off unless you do that homework," Lewis said.

"We're very confident they spent a fair amount of time at the banks. They knew what time the couple was home each night. Those details you don't leave to chance," Lewis said.

People with information about the robbery are urged to call the Phoenix FBI Field Division at 602-279-5511 or their local authorities. Callers are not required to identify themselves.

As the manhunt for the robbers continues, Tempe Chief of Police Tom Ryff had some advice for Valley bank employees.

"Beware of your surroundings," he said.

"Look for suspicous activity and if you see or hear something ordinary, call authorities," Ryff said.

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