BEIJING — When Hu Jintao, China’s top leader, picked up the telephone last August to talk to a senior anticorruption official visiting Chongqing, special devices detected that he was being wiretapped — by local officials in that southwestern metropolis.

The discovery of that and other wiretapping led to an official investigation that helped topple Chongqing’s charismatic leader, Bo Xilai, in a political cataclysm that has yet to reach a conclusion.

Until now, the downfall of Mr. Bo has been cast largely as a tale of a populist who pursued his own agenda too aggressively for some top leaders in Beijing and was brought down by accusations that his wife had arranged the murder of Neil Heywood, a British consultant, after a business dispute. But the hidden wiretapping, previously alluded to only in internal Communist Party accounts of the scandal, appears to have provided another compelling reason for party leaders to turn on Mr. Bo.

The story of how China’s president was monitored also shows the level of mistrust among leaders in the one-party state. To maintain control over society, leaders have embraced enhanced surveillance technology. But some have turned it on one another — repeating patterns of intrigue that go back to the beginnings of Communist rule.