During the robbery, which took place just two days after the gang escape, the two men stole police radio scanners that were later used during a Christmas Eve holdup in which a police officer was killed.
The seven-member gang was already wanted in connection with their prison break, the robbery of a sporting goods store in Irving, Texas, and the murder of police officer Aubrey Hawkins, 29. Hawkins was killed when he responded to a robbery call from the store.
On Friday, police charged George Rivas, 30, and Donald Keith Newbury, 38, with the aggravated robbery of the Radio Shack store in the Houston suburb of Pearland on December 15.
Authorities have offered a $100,000 reward for information that leads to the gang's capture.
Stole police scanners
Police allege that Rivas and Newbury tied up Radio Shack store employees at gunpoint, grabbed items including police radio scanners, and then fled in the store manager's truck, which was found later at a nearby apartment complex.
Those scanners may have played a role in the Oshman's sporting goods store robbery that led to Hawkins' death.
Irving police said the gang, posing as security guards and carrying scanners, corralled two dozen store employees and then stole guns, about $70,000 in cash, and clothes.
Hawkins was gunned down when he came upon the gang as it fled the store, police said. The fugitives also stole his gun and ran him over with the stolen truck, police said.
Hawkins, who had been an officer since last October, was buried Thursday in Irving. He was the 14th police officer killed this year in Texas, which leads the nation in such deaths.
Why the gang needed more weapons seems unclear. At the time of their escape, they stole 14 .357 Magnum pistols with 238 rounds of ammunition, a loaded automatic rifle, and a loaded shotgun.
Police fear the fugitives do not intend to be stopped, considering that they would face the death penalty if convicted of Hawkins' murder.
"Given the history of the kind of people we're dealing with, the likelihood of having another serious injury or death is a real good possibility," Irving Police Chief Lowell Cannaday told CNN.
Ringleader has 'street smarts'
Similarities between the Oshman's robbery and six earlier ones committed by Rivas have led authorities to believe that the 30-year-old kidnapper is the gang's mastermind.
"He has street smarts and is very persuasive," said Larry Todd, spokesman for the Texas Department of Criminal Justice.
All seven escaped inmates were serving terms of 30 years or more for violent crimes. Two of the men are convicted murderers. Others were serving time for robbery, rape, and assault and injury to a child.
Their cohesion has made capturing them more difficult, Todd said.
"If they would split up, our chances of getting them would increase. Right now, they're sharing information, encouraging each other and not doing the normal things escapees do," Todd said.
Rivas was serving 99 years for aggravated kidnapping and burglary. Prosecutors and law officers called him "one of the most dangerous men in El Paso."
Rivas was caught after a string of armed robberies in El Paso, Texas, which resembled the Christmas Eve robbery and included the same target -- an Oshman's Super Sports USA store. His arrest followed a three-hour standoff with police.
The seven overpowered a guard and several civilian overseers while working near Kenedy, in south Texas, on December 13. They fled in a state-owned pickup truck to a rendezvous at a Wal-Mart store in Kenedy.
Outside accomplices, whom investigators have yet to identify, met them with two cars.
The Associated Press and Reuters contributed to this report.
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