Jan 16 (Reuters) - FBI leaders have warned that hackers

who breached AT&T's ( T ) system last year likely stole months

of agents' call and text logs, prompting an urgent effort to

safeguard confidential informants' identities, Bloomberg News

reported on Thursday.

The breach, believed to have compromised all FBI devices

using the bureau's AT&T ( T ) public safety service, included agents'

mobile phone numbers and the numbers they used to call and text,

according to a document reviewed by Bloomberg and its interviews

with a current and a former law enforcement official.

Last July, AT&T ( T ) said that the company suffered a massive

hacking incident as data from about 109 million customer

accounts containing records of calls and texts from 2022 was

illegally downloaded in April.

FBI officials informed agents nationwide that details about

their use on the telecom carrier's network were likely among

billions of records stolen, the report said, adding that while

the hacked records did not expose the content of communications,

they could link investigators to their secret sources, the

report added.

AT&T ( T ) and the FBI did not immediately respond to Reuters'

requests for comment.

The breach follows broader concerns about cyber-espionage

targeting U.S. telecom networks. On Jan. 10, U.S. national

security adviser Jake Sullivan said that the U.S. had taken

steps in response to Chinese-linked cyber-espionage operations

against U.S. telecoms firms.

U.S. telecoms firms Verizon and AT&T ( T ) said late last

year their networks had been targeted by the cyber hackers, but

were now secure as they worked with the U.S. government and law

enforcement.