The investigation into the April 23 violence concluded that "the terrorists had no connection with foreign forces," Xinjiang government spokesman Hou Hanmin was quoted as saying by the China Daily newspaper.
China has said the assailants, 25 of whom have either been killed or arrested, were inspired by jihadi propaganda and were planning a major attack before being discovered by local government workers.
Hou's statement raises the specter of a home-grown terrorist movement inspired by calls for an Islamic holy war, but not directly connected to or acting under the orders of overseas-based separatists or insurgent groups. That appears to challenge official claims that violence in Xinjiang is not a result of ethnic or religious tensions, but is the work of meddling outsiders seeking to destabilize the region. Xinjiang's native Muslim Turkic Uighur population is culturally, religiously, and linguistically distinct from China's Han majority, and many Uighurs claim they are being marginalized and oppressed by Han migration and heavy-handed rule by Beijing.
China has consistently linked Xinjiang violence to the global jihadi movement and overseas Uighur activists, but has provided little hard evidence.
Police earlier said the men in last week's attack held secret Quran study sessions and possessed extremist religious literature and flags bearing jihadi slogans, providing justification for its strict rules on Islam in Xinjiang. The men killed 19 police and community workers after their bomb-making materials were discovered, then attacked local government offices and a police station. Official accounts appear to show that those slain were unaware of the men's intentions and underestimated the level of risk.
A leading Uighur group based in Germany has called for an independent investigation into the attack, saying China has consistently failed to provide convincing evidence for its terrorism claims that it uses to justify for tougher security measures.
"The World Uyghur Congress urges the international community to uphold its pressure on China in an effort to see Beijing stop applying its same pattern of information black out, arbitrary detentions and forced disappearances after each incident in the region," said a statement from the group, which uses an alternative spelling for Uighur.
A sprawling region that borders Central Asia, Afghanistan and Pakistan, Xinjiang sees recurrent violence pitting members of the Turkic Muslim Uighur (pronounced WEE'-gur) group against the authorities and Han migrants. Among other restrictions, China imposes strict rules on Uighur religious life, including barring children and government employees from mosques, ordering young men to trim their beards and banning the wearing of veils by women.
Beijing says it treats minorities fairly and spends billions of dollars on improving living standards in minority areas.