Bus driver Mynor Gonzalez ignored the threats: "200 quetzales a week, or we'll kill you." He knew drivers who didn't pay the roughly $24 "protection" fee had been murdered, but always on other routes. He knew his job _ once considered secure and well-paying for Guatemala's poor _ had become deadly.

Then it happened on his route. The gangs used his friend, driver Miguel Angel Chacon, 34, to show they meant business.

"He saw them approaching the bus, so he jammed on the brakes and started running toward the back," said Gonzalez, 30. "They shot him twice in the back, right there in the aisle in front of all the passengers."

Gunmen have killed more than 170 bus drivers this year to scare them and transportation companies into paying extortion fees that fuel the country's multimillion-dollar organized crime network.

It's a small number of deaths given Guatemala's roughly 6,200 murders a year _ a homicide rate that puts the Central American country among the world's 10 most dangerous, according to U.N. crime studies. But the public execution of bus drivers _ often witnessed by as many as 50 passengers _ adds a new level of brutality to an already terrorized nation.

There is no viable public transit in Guatemala City outside of the 8,000 buses that carry about 1 million people daily in this capital of 3 million. Passengers have no choice but to ride.

"You are always scared that the bus you take is the one that they will target and maybe you will get a stray bullet," said Damaris Lopez, 21, a student who regularly takes a route where 10 drivers have been shot. "But I don't have a car. How else can I get around?"

Drivers and transportation companies have no choice but to pay.

Congressman Anibal Salguero owns a bus company and says he shells out an average of about $60 a week per bus.

"I'm a congressman, I could have them arrested," Salguero said. "But then what? Have the gangs take it out on my drivers?"

Even jailing doesn't work. Gangs run extortion rings from their cells, said Rony Lopez, a prosecutor who heads the organized crime unit.

"They smuggle in cell phones, pay or terrorize prison guards into turning off the signal blockers and have people working outside to collect the money and carry out the murder of drivers," Lopez said.

It started small about 5 years ago, with gang members extorting $1 to $2 a day in protection fees on individual routes. But it quickly grew into an organized racket once criminals realized they could rake in hundreds of thousands of dollars.

At first, assassins would hop a crowded bus, insult the driver and then shoot him in the head to set an example.