It was the third deadly attack in nine days in Xinjiang, an oil- and gas-rich region on the border with Afghanistan, Pakistan and six Central Asian nations where Islamic separatists have fought the central government for years.
No one has claimed responsibility for the knife attacks. Government officials call them terrorism and insist they have nothing to do with the Olympics. They have not produced any evidence to back up their claims.
Experts say at least one of the recent attacks signaled a new level of sophistication for the region, but that there was no enough information known about them to say with any certainty who was behind them.
In the latest violence, an unknown number of attackers leaped out of a vehicle at a checkpoint in Yamanya town and stabbed four government employees who were taking down the names of people who passed through, said Tu'ersenjiang, an officer at a police post in the town who would only give one name.
Three of the guards died at the scene, and the fourth was hospitalized in critical condition.
"He has pulled away from danger," said a man who answered the phone at the public security bureau in Shule country, where Yamanya is located. "We are now waiting for him to wake up and speak so we can find out more details about what happened."
The state-run Xinhua News Agency said the attack occurred about 9 a.m., and that the attacker or attackers escaped.
Tu'ersenjiang said police were investigating. The guards were not members of the police or military, he said.
It was the third attack on government-linked guards this month in Xinjiang, which is dominated by China's Muslim ethnic minority Uighurs.
On Sunday, militants tossed homemade bombs at government buildings in Kuqa city, 435 miles (700 kilometers) northeast of Yamanya, then fought with police. Twelve people died, officials said, including 10 assailants.
Six days earlier, attackers rammed a truck into a group of border police then attacked them with knives and homemade bombs in Kashgar, a town neighboring Yamanya.
Residents and experts say many Uighurs are seething with anger toward Chinese immigrants whom many here see as symbols of government oppression.
Anti-government violence has flared sporadically in Xinjiang for years. But Nicholas Bequelin, a researcher with New York-based Human Rights Watch's Asia Division, said Sunday's attacks - 13 men and two women used an explosive-laden motorized tricycle and lobbed bombs at 17 sites in the city in quick succession - appeared more highly organized.
"It presents several new aspects which were not present in previous incidents in Xinjiang," he said. "One is the sophisticated coordination of the attacks: It was not just one attack. It's a string of bombings that requires much more planning and a larger organization to carry out especially at the time of the Olympics when the security is so high."
Many Uighurs declined to talk to The Associated Press on the record about the situation in Xinjiang, though some said privately there was widespread anger at the Chinese and at the government for oppressing the minority and their religion.
Security was tightened in the region for the Olympics, and again after the attacks.
Some officials are openly hostile to press. An AP reporter and photographer waiting in line to register at a police post outside Korla town on Tuesday were approached by a policeman wearing a flak jacket and helmet, who pushed the reporter back and pointed his assault rifle at him.
"This is Chinese territory. You have to obey the rules," he said, keeping the gun pointed at the reporter as he glared at him for five minutes. After registering, the journalists returned to their vehicle and continued their journey.
Dilxat Raxit, a spokesman for the Germany-based, pro-independence World Uighur Congress, claimed in an e-mail received Tuesday that more than 90 Uighurs have been detained recently, and some had been tortured.
Kuqa county chief Yusufujiang Maimaiti said "there was not an ounce of truth" to Raxit's allegations, but he would not say whether any arrests had been made.
