Suspicious behavior on T-Mobile US Inc.’s network devices tipped off the company to a breach that was potentially part of a sprawling cyber-espionage campaign that has raised urgent questions about the exposure of a critical sector of the economy.

T-Mobile says the Chinese "Salt Typhoon" hackers who recently compromised its systems as part of a series of telecom breaches first hacked into some of its routers to explore ways to navigate laterally through the network.

T-Mobile has severed its connection to the unnamed wireline provider, which it suspects may still be compromised.

The mobile carrier provided its first detailed rundown of how it stopped hackers from infiltrating its networks.