Three environment protection officials in Xi'an, capital of Shaanxi province, have been detained for allegedly tampering with air quality monitoring data.

According to police sources, the suspects are the director of Xi'an Environment Protection Administration's Chang'an branch, the chief of an air monitoring station in the Chang'an district of the city and his deputy.

They are in custody in a local detention center awaiting further investigation.

The suspects are accused of having a duplicate key cut without authorization so that they could access an air monitoring station and block the equipment with yarn to lower the amount of pollution it could detect.

It is thought the trio began tampering with the equipment in February, in a bid to artificially improve the district's air quality readings, so as to avoid punitive action.

The air monitoring station is one of two sub stations in the city directly administered by the Environmental Protection Ministry, according to insiders, who said the affects could be wide-ranging if the monitoring data was found to be fraudulent.

A company in Wuhan, Hubei province, had been entrusted to maintain the facilities.