The data of nearly all customers of U.S. telecommunications giant AT&T was downloaded from a third-party platform in a security breach, the company said Friday.

The breach, which took place in April of this year, hit AT&T's cellular customers and customers of mobile virtual network operators using AT&T's wireless network, as well as landline customers who interacted with those cellular numbers.

Approximately 109 million customer accounts were impacted, according to AT&T, which said that it currently doesn't believe that the data is publicly available.

The compromised data also doesn't include some information typically seen in usage details, such as the time stamp of calls or texts, the company said, or customer names. AT&T, however, said that there are often ways of using publicly available online tools to find the name associated with a specific telephone number.

An internal investigation determined that compromised data includes AT&T records of calls and texts between May 1 and October 31, 2022.

Compromised data also includes records from January 2, 2023, for a very small number of customers.

AT&T identified the third-party platform as Snowflake and said that the incident was limited to an AT&T workspace on that cloud company's platform and did not impact its network.

The Federal Bureau of Investigation (FBI) said that it has worked collaboratively with AT&T and the Justice Department "through the first and second delay process, all while sharing key threat intelligence to bolster FBI investigative equities and to assist AT&T's incident response work."

The Department of Justice (DOJ) said Friday that it became aware of the breach early this year, but that it met the security standard for a delayed filing by AT&T with the U.S. Securities & Exchange Commission, a filing that was made public Friday.

The DOJ said an earlier disclosure of the breach would "pose a substantial risk to national security and public safety."

The Federal Communications Commission is also investigating.