The Trump campaign was informed this week that hackers may have gained access to data from the phones through a breach of American telecommunications systems.

Chinese hackers who are believed to have burrowed deep into American communications networks targeted data from phones used by former President Donald J. Trump and his running mate, Senator JD Vance of Ohio, people familiar with the matter said on Friday.

Investigators are working to determine what communications data, if any, was taken or observed by the sophisticated penetration of telecom systems, according to these people, who spoke on the condition of anonymity to describe an active and highly sensitive national security case.

The type of information on phones used by a presidential candidate and his running mate could be a gold mine for an intelligence agency: Who they called and texted, how often they communicated with certain people, and how long they talked to those people could be highly valuable to an adversary like China. That sort of communications data could be even more useful if hackers could observe it in real time.