Rep. Ted Lieu (D-Calif.) is calling on the Federal Communications Commission (FCC) to accelerate its investigation of the Signal System 7 (SS7) flaw.

In a public letter to FCC Chairman Thomas Wheeler, Lieu asked the agency head to “expedite” the ongoing investigation into the vulnerability. He warned that the “problem is no longer a theoretical threat,” noting that a breach of the Democratic Congressional Campaign Committee (DCCC) earlier this month released mobile phone numbers of House Democrats.

The SS7 flaw, discovered by a German researcher Karsten Nohl, affects the technology that allows cell phones to share information with each other and with mobile carriers.

“We now have a mass release of cell phone numbers of Members of Congress likely caused by a Russian government that has full access to utilize the SS7 flaw,” Lieu wrote. “Because we don't know how long the hackers had access to this information, it is very possible nearly half of Congress already had voice and text data intercepted.”