Apple told Donald F. McGahn II, the White House counsel to former President Donald J. Trump, last month that the Justice Department had subpoenaed information about an account that belonged to him in February 2018, and that the government barred the company from telling him at the time, according to two people briefed on the matter. Mr. McGahn’s wife received a similar notice from Apple, said one of the people, who spoke on the condition of anonymity to discuss a sensitive matter. It is not clear what F.B.I. agents were scrutinizing, nor whether Mr. McGahn was their specific focus. In investigations, agents sometimes compile a large list of phone numbers and email addresses that were in contact with a subject, and seek to identify all those people by using subpoenas to communications companies for any account information like names, computer addresses and credit card numbers associated with them.