BEIJING, July 2 -- China's securities watchdog announced late Thursday it will investigate suspected manipulation of the stock market as experts fail to explain worst collapse in years.

Zhang Xiaojun of the China Securities Regulatory Commission (CSRC), said the investigation will focus on such activities occurring simultaneously in multiple markets. The CSRC has tracked irregularities between securities and futures trading.

The CSRC will transfer any criminal cases to the police, Zhang said.

The announcement came as the losing streak in China's A-share market continued with wild speculation on the reasons behind the stampede, with some even pointing to conspiracy.

China Financial Futures Exchange on Wednesday denied a rumor that overseas investors have been shorting A-shares via stock futures.

Shares continue their descent on Thursday. The benchmark Shanghai Composite Index fell 3.48 percent to finish at 3912.77 points.