Suspicious behavior on T-Mobile US Inc. network devices. exposed the company in a breach that may have been part of an extensive cyberespionage campaign that has raised urgent questions about the exposure of a crucial sector of the economy.

Jeff Simon, T-Mobile’s chief security officer, said in an interview with Bloomberg News that the behavior, while not “inherently malicious,” was unusual enough to attract the attention of the company’s network engineers. In recent weeks, engineers had noticed unauthorized users executing commands on the company’s network devices, which appeared to prompt them to probe the network’s structure, Simon said.