DALLAS (AP) — Texas police were wearing bulletproof vests and increasing their firepower in the biggest manhunt in state history — a search for seven escaped convicts believed to be using stolen police scanners to stay a jump ahead of the law.

In the three weeks since they broke out of prison with a cache of weapons and left a note warning, "You haven't heard the last of us yet," the convicts are suspected of pulling off at least two robberies and killing a police officer.

Prison officials said the search has spread across the Southwest, but is concentrated in the Dallas area. Larry Todd, a spokesman for the Texas Department of Criminal Justice, said all seven are still believed to be together.

Todd said they apparently have not tried to contact any close friends or relatives, but he added: "Someone is obviously helping them. They have the money and assistance to stay hidden."

That the seven appeared to be running as a gang and instead of splitting up has led to suspicions they may be planning a showdown.

"It leads everyone to think these guys have a plan," said FBI spokeswoman Lori Bailey.

The U.S. Bureau of Alcohol, Tobacco and Firearms in San Antonio charged the men with weapons violations Wednesday, signaling a broadening federal role in the three-week-old manhunt, the San Antonio Express-News reported.

FBI agents in the Dallas area planned to file separate federal charges of unlawful flight to avoid federal prosecution.

The unlawful-flight charges would "make it a nationwide manhunt rather than a state and local manhunt," Bailey said.

Officials said the gang has amassed an arsenal of more than 40 weapons stolen during the prison break in Kenedy on Dec. 13 and a robbery at a sporting goods store on Christmas Eve in which an Irving police officer was shot to death.

The FBI has drawn up psychological profiles of the seven, and investigators believe the ringleader is George Rivas, 30, who was serving a life sentence for aggravated robbery and kidnapping and had been implicated in more than a dozen robberies since 1987 in the Southwest.

Investigators have portrayed Rivas as fearless and shrewd with a powerful hold over the others.

All seven convicts — two killers, two armed robbers, a child abuser, a serial rapist and a burglar — are being sought on capital murder charges for the police officer's slaying.

An autopsy report released Wednedsay showed Irving Officer Aubrey Hawkins suffered 11 gunshot wounds and was run over by a vehicle after he responded to the sporting goods store robbery.

Hawkins was shot six times in the head and the shots were fired from different directions. The report doesn't identify whether investigators think more than one gunman fired or which of the escapees are suspected.

Rivas and Donald Newbury, 38, also serving time for aggravated robbery, were identified in photo lineups by victims of a Dec. 15 robbery of a Houston-area Radio Shack store, where the scanners were stolen. Investigators believe the gang may be using the scanners to monitor police.

"That does provide an advantage for them," Todd said. "They are using patience and diligence. Our law enforcement must be equally as patient and diligent."

The FBI, U.S. Bureau of Alcohol, Tobacco and Firearms, and Texas' Departments of Public Safety and Criminal Justice have investigated hundreds of leads. A $100,000 reward has been offered for the convicts' arrest.

The Criminal Justice Department posted family-provided photos of some of the seven on its Web site along with descriptions of vehicles they may be using.

Law enforcement agencies across the region have studied the photos and are on high alert when responding to any calls or making traffic stops, in part because of the police officer's slaying.

Capt. Robert Flores of the police department in the Dallas suburb of Allen said officers are briefed daily about the convicts, whose photos are posted "so they can constantly keep them in mind." Most uniformed officers in the 67-member department are wearing bulletproof vests, he said.

Thomas Lauer, chief of the 12-member Wilmer police department, said his officers in the Dallas County town are carrying an extra shotgun.

"Officers are carrying a little extra firepower in their vehicles. Instead of radio checks every five to seven minutes, we're checking every three minutes to make sure an officer is OK," he said.

The breakout was the biggest escape from a Texas state prison in modern history. Texas has recaptured all but one of the 143 inmates who escaped from its state prisons in the past 10 years. Most were inmates who walked away from work duty or escaped alone or with one or two others.

Two years ago, seven Texas death row inmates tried to escape, but only one made it over the wall. His body was found in a river on prison grounds a week later.