DALLAS (AP) — Police believe an armed robber described as a master criminal is leading a band of heavily armed escaped inmates accused of killing a police officer, and authorities fear the group may be planning a violent showdown.

"They are apparently on a mission, and it's a pretty dark mission," FBI spokeswoman Lori Bailey told The Dallas Morning News. "Does anybody know what that is right now? I don't think anybody does."

Bailey said the federal agency was compiling behavioral profiles of the escaped convicts.

The only clue to the plans of the seven inmates, which includes two murderers, was a note left during their escape from a Kenedy prison on Dec. 13: "You haven't heard the last of us."

The last anyone has apparently seen of them was a robbery at an Irving sporting goods store on Christmas Eve that left a police officer dead. The bandits got away with more than two dozen weapons, including semiautomatic hunting rifles and pistols.

When they broke out of prison, they escaped with an arsenal of weapons taken from a guard tower.

Those weapons and rumors that they are being aided raises the specter of a violent standoff, Bailey said.

"Certainly we're all concerned about that," she told the Los Angeles Times. "They have quite a cache of weapons and ammunition, and they're obviously not afraid to use them."

Authorities believe gang leader is George Rivas, who 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, 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 slain rookie police officer, 29-year-old Aubrey Hawkins, was remembered Thursday as a kind, helpful man whose lifelong dream was to make the world better by fighting crime.

"It's not how Aubrey died that made him my hero — it's how he lived," Irving Police Chief Lowell Cannaday told the more than 1,000 people packed in the Calvary Temple.