Hundreds of members of Congress may have had all of their cellular voice and text data intercepted by foreign governments before a leak of their information came to light this month, Rep. Ted Lieu said Wednesday.

The California Democrat made the point in a letter asking Federal Communications Commission Chairman Tom Wheeler to pick up the pace of an investigation into possible surveillance that Wheeler said the agency began in April. The surveillance is made possible by a flaw in a global telecommunications network known as Signaling System 7, which allows intelligence agencies to hack into any telephone number they’re able to plug into a computer.