(Bloomberg) -- China’s main stock exchanges have cracked down on speculation involving the trading of shares of two firms, underscoring the nation’s efforts to curb a practice that the securities chief once called a “toxic tumor.” 

The “relevant” accounts that were involved in predicting the stock moves of Nanjing Chemical Fibre Co. and Zhongtong Bus Co. have been suspended for 15 days for spreading “fabricated” information, according to statements Tuesday from the Shanghai Stock Exchange and the Shenzhen Stock Exchange. The share prices fluctuated in volatile trading after the speculation.    

The price swings may have been coincidental but the speed with which the messages went viral highlights the enormity of the task confronting regulators as they vow to tighten the enforcement of measures against stock manipulation. The volatility comes at a tricky time as the world’s second-largest equity market is attempting to consolidate its rebound after concerns about China’s growth outlook fueled a selling spree.

Nanjing Chemical fell by the 10% daily limit on May 20, when there was a posting on an investor forum early afternoon that the company “will have a surprise.” Shares then spiked by the daily limit 10% a day later. 

On Tuesday, Zhongtong Bus shares similarly jumped 10% after someone posted a message that read “I’m the banker and the stock will rise by the daily limit at 1:20.”

The Shenzhen exchange eventually revealed in a statement that an investor named Chen Guo wrote the post, and that he held 700 Zhongtong shares prior to May 21. Two hours before his online claim, the investor placed an order to offload that entire stake at the up-limit, according to the statement. The order was canceled while the stock was rising, it said. 

“The exchanges should investigate the truth and provide a conclusion on whether these predictions and their fulfillment are pure coincidence, or a case of market manipulation in public provocation,” according to a front-page report in the Securities Times. “Should it be the latter, these events of such vile nature that damage the market’s just, fair and open order should be punished with utmost severity.” 

About two-thirds of Nanjing Chemical Fibre’s free-float shares changed hands Tuesday. The company, which manufactures yarns and cellulose fiber, said in a statement Wednesday that “there may be irrational speculation” in its shares. It also reminded investors that its business is not engaged in the “low-altitude” economy, a drone- and other air delivery-based sector whose shares have risen in recent weeks.  

Zhongtong Bus also said in a statement Tuesday that its “recent production and operation conditions are normal.” 

“After self-examination, the company did not violate fair information disclosure,” it said. 

--With assistance from Helen Sun.

(Updates with details from Shenzhen exchange in sixth paragraph.)

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