The driver was treated for a cut to his neck and was stable after surgery, a hospital official said. The attacker, who had a Croatian passport, was killed, the FBI said.
Speaking by cellular phone from the crash site, passenger Carly Rinearson told CBS affiliate WTVF that a man who looked to be 30 to 35 years old kept approaching her front seat in the bus and asking what time it was. The man then asked if he could have her seat, Rinearson said.
She said no. Afterward, the man walked up to the front of the bus and slashed the driver's throat, she said.
"He just went up to the bus driver and, like, slit his throat, and the driver turned the wheel and the bus tipped over," Rinearson said.
She said the bus then swerved off the road and crashed.
The crash happened on Interstate 24 near Manchester, 50 miles southeast of Nashville. The bus originated in Chicago with a final destination of Orlando, Fla., Greyhound spokesman Mike Lake said.
Six people died at the scene, and the 34 others on board were injured, said Dana Keeton, a Tennessee Department of Safety spokeswoman.
Greyhound initially said 10 people had died, but the company's chairman later told reporters that six had died.
Keeton said the injured were taken to at least six hospitals. Hospital officials described the injuries as ranging from bumps and bruises to some that required emergency surgery.
After the 5:15 a.m. EDT crash, Greyhound pulled the 2,000 to 2,500 buses operating at the time off the nation's highways, but after consulting with federal and state investigators and transportation officials, the company decided it was safe to resume service as of 1 p.m. EDT.
"The officials have assured me that they believe this tragic accident was the result of an isolated act by a single deranged individual," Greyhound president and CEO Craig Lentzsch told reporters in Washington.
Earlier, U.S. Justice Department officials said they did not believe the attack was terrorist-related, but that the investigation was continuing.
Coffee County Medical Examiner Dr. Al Brandon said the driver told him the attacker had boarded the bus in Kentucky. He said the man, who had been polite and spoke with a foreign accent, got up several times to ask him where the bus was headed, Brandon said.
The driver, whose name was not immediately released, told Brandon the passenger then "accosted" him with a box cutter.
However, Brandon said he couldn't confirm the weapon was a box cutter, saying it was a "sharp instrument similar to a razor blade." The terrorists who hijacked four airliners on Sept. 11 reportedly used box cutters in their suicide attacks.
After attacking the driver, the passenger grabbed the steering wheel, orcing the bus into the oncoming lanes of the interstate before it crossed the road and tipped over onto its right side, the medical examiner said.
The driver was able to crawl from the wreckage through a window and tried to flag down passing vehicles. He told Brandon the attacker was thrown through the windshield.
The bleeding driver managed to climb out of the overturned bus and was in good condition. Dr. Ralph Bard of Manchester Hospital quoted the driver as saying "the suspect never acted threatening until the actual attack," after boarding the bus in Louisville, Ky.
The bus, No. 1115, left Louisville, Ky., and was due to stop next in Atlanta, Greyhound spokeswoman Karen Chapman said.
Dallas-based Greyhound, which carries about 70,000 passengers a day, stopped all service as a precaution after the crash, spokeswoman Kristin Parsley said. She said buses already en route were allowed to continue to their destinations.
Greyhound carries about 25 million passengers a year as the last remaining nationwide bus service.
Lentzsch said Greyhound was offering full refunds to passengers who decided against taking trips. He also said Amtrak agreed to accept Greyhound bus tickets.
He added that security was being bolstered.
"Prior to reboarding passengers today, we are hand searching carry on luggage," Lentzsch said. He also said some passengers have been checked with a wand used to detect metal devices.
Passengers across the country, already jittery after last month's terror attacks, had to wait hours or find other means of transportation.
"People are a little panicky about it," said Joi Smith, a Greyhound agent in New Hampshire. "They are freaked out, which is understandable."
Greyhound had begun boosting security in many terminals around the nation, said Tim Barham, district manager of driver operations in Washington.
"Ever since the September 11 events we've had several discussions and started to implement extra security," he said.
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