$VZ $LUMN U.S. Officials Probe Severity of China-Tied Cyberattacks to Broadband Providers -- WSJ Hackers Maintained Access to Some Breached Networks Within Last Week, Sources Say -- WSJ More Potential Victims Have Been Notified of Possible Compromise, Sources Say -- WSJ Lawmakers Send Letters to Verizon, AT&T, Lumen Requesting Information on Hacks, Responses -- WSJ