It is not clear precisely who has been affected by the latest attacks. Mandiant, a private security company that helps companies and government agencies defend themselves from hackers, said the attacks had resumed but would not identify the targets, citing agreements with its clients. But it did say the victims were many of the same ones the unit had attacked before.

The hackers were behind scores of thefts of intellectual property and government documents over the past five years, according to a report by Mandiant in February that was confirmed by American officials. They have stolen product blueprints, manufacturing plans, clinical trial results, pricing documents, negotiation strategies and other proprietary information from more than 100 of Mandiant’s clients, predominantly in the United States…

Mandiant said that the Chinese hackers had stopped their attacks after they were exposed in February and removed their spying tools from the organizations they had infiltrated. But over the past two months, they have gradually begun attacking the same victims from new servers and have reinserted many of the tools that enable them to seek out data without detection. They are now operating at 60 percent to 70 percent of the level they were working at before, according to a study by Mandiant requested by The New York Times.