WITHIN HOURS OF the storming of the Capitol on January 6, the FBI began securing thousands of phone and electronic records connected to people at the scene of the rioting — including some related to members of Congress, raising potentially thorny legal questions.

Using special emergency powers and other measures, the FBI has collected reams of private cellphone data and communications that go beyond the videos that rioters shared widely on social media, according to two sources with knowledge of the collection effort.

In the hours and days after the Capitol riot, the FBI relied in some cases on emergency orders that do not require court authorization in order to quickly secure actual communications from people who were identified at the crime scene. Investigators have also relied on data "dumps" from cellphone towers in the area to provide a map of who was there, allowing them to trace call records — but not content — from the phones.