Three months after the fierce outbreak of ethnic violence in the Western Chinese region of Xinjiang, when more than 200 ethnic Chinese and predominantly Muslim Uighurs were killed, our world affairs editor John Simpson has visited Xinjiang's two main cities, Urumqi and Kashgar.
Urumqi is a city under siege - there are patrols of soldiers and armed police in full riot gear everywhere.
In a 10-minute walk along the city streets you are likely to encounter four or five of them, each composed of a dozen or so men.
The tension is evident - few people are prepared to speak about what happened here, and none openly.
One Uighur woman spoke to us in secret about the events of 5 July.
She witnessed the murder of two ethnic Han Chinese by a gang of Uighurs.
"People were going crazy," she said. Altogether, 198 Chinese died that day.
Then, two days later, Chinese gangs carried out revenge killings of Uighurs.
No official figures have been issued, but the woman thought about 10 Uighurs had been killed.
The authorities are very nervous about the presence of foreign journalists.
Access to ordinary people was limited by authorities
Everywhere we went in Urumqi my television team and I were followed, sometimes by three unmarked police cars at a time.
And when we flew on to Kashgar, where many of the more militant Uighurs involved in the riots came from, the police detained us at the airport.
We were allowed to stay in Kashgar until the next morning, but everywhere we went a contingent of police followed us and prevented our filming or interviewing anyone.
It was clear they thought we had come to meet Islamic fundamentalists, and were determined to stop us.
That night, we were kept under house arrest at a hotel in the centre of Kashgar.
Ethnic violence is something that worries the Chinese government deeply. It threatens the cohesion of the entire country.
The immediate cause of July's rioting seemed small enough - rumours spread that two Uighur workers had been killed by Han Chinese in south-east China, thousands of miles away.
Yet the hostility towards Han Chinese which many Uighurs in Xinjiang feel is so intense that trouble broke out at once.
The origins for this hostility are complex. The Chinese government has often treated Uighurs generously, offering promising students places at good universities and making it easy for them to work elsewhere in China.
Yet many remain wretchedly poor. Now the poverty-stricken areas of cities like Urumqi and Kashgar are being knocked down and new housing is being built, but this often increases local resentment.
People see it as a direct attack on their traditions and culture.
Over the years, ethnic Chinese immigration into Xinjiang has sometimes been encouraged by Beijing and sometimes not, but the net result is that in Urumqi, their own capital city, Uighurs are now a minority.
There are increasing signs of separatist feeling among them. The discovery of oil has convinced many Uighurs that if they were independent, they could be a viable state.
Most disturbing of all for the Chinese authorities, though, is the growing influence of extremism.
Uighurs say it scarcely existed before the mid-1990s, and that China was slow in waking up to the challenge.
Now there are plenty of mosques, particularly in Kashgar, where fundamentalist Uighur imams are active.
The Chinese Communist Party, always nervous when any rival organisation or movement starts to attract support, has responded with the creation of new task forces.
Known as "social stability teams", they act partly as social security workers, addressing grievances, and partly as the eyes and ears of the authorities. Many Uighurs have been recruited to the teams.
We came across some of them in the slum area of Gulistan, a Uighur stronghold in Urumqi, as they were going from door to door.
They work closely with the undercover police, and in Gulistan they co-operated with the eight or more in plain clothes who were following us around.
Urumqi itself is quiet now. The big deployment of soldiers and police has ensured that.
But in Kashgar the authorities seem far less confident. Three months after the rioting, it is all too clear that the Chinese authorities have not yet got the situation under full control.
And they are plainly worried.