Three fugitive siblings who fired at a police officer before robbing a bank in Georgia have been "very clear in demonstrating their threat to both law enforcement and the public," an FBI agent told CNN.

Dubbed the Dougherty Gang, the three siblings are believed to be deeply loyal to each other and capable of extreme violence.

"It's very unusual to have siblings like this so well armed, so committed to whatever mission that they think they're on and to have demonstrated such propensity for violence," FBI Special Agent Stephen Emmett said Monday on "Good Morning America."

"The problem that we feel we're facing is that they're committed to each other as siblings, so that's going to create additional issues for any law enforcement that is confronting them."

Ryan Edward Dougherty, 21 years old; his sister Lee Grace Dougherty, 29; and half-brother Dylan Dougherty Stanley, 26 -- have 20 felonies among them, authorities said. Most recently, Ryan Dougherty was forced to register as a sex offender based on the content of text messages he sent to an 11-year old girl.

The family members shot at a police officer in Florida on Aug. 2 after the officer attempted to pull them over for speeding, authorities said. A few hours later, they fired more shots during an alleged armed robbery of a bank in Georgia.

"I'm not going to compare them to Bonnie and Clyde -- I'll let you do that if you like," Emmet told CNN Saturday.

"It's not very often that we have advance warning like this of potential problems with law enforcement encountering individuals. They made it very clear that they are going to be a problem," he added.

The fugitives' mother, Barbara Bell, begged her children to surrender.

"Only mom knows what good people you are inside," Bell told an ABC news affiliate in Tampa-St. Petersburg. "Please prove me right and everybody wrong by doing the right thing now and turning yourselves in.

"Your lives are not over. You've made some bad mistakes, but so far no one has been physically harmed," Bell said. "The only safe thing, the only right thing, the only good thing to do is to turn yourselves in."

Both law enforcement and those who know the fugitives are pessimistic about the possibility of a positive outcome.

ABC News reported that the siblings sent their mother a text message that read, "There's a time for all of us to die."

"They are well armed. They have at least an AK-47 assault rifle and a MAC -10 or MAC -11 type machine pistol. These are high-capacity, high-rate-of-fire weapons that are normally seen by the military or law enforcement. These weapons are very damaging weapons and they've shown that they will fire them," Emmett told ABC.

The fiance of Lee Grace Dougherty told the media, "It's hard for me to see this thing turning out happily."