SAN FRANCISCO (CBS SF) — Traffic was flowing freely on the San Francisco-Oakland Bay Bridge again early Friday after police activity shut down several eastbound lanes for more than four hours, triggering a massive backup.

The California Highway Patrol said the backup was not related to a crash, but did not disclose what the issue was that had forced the closure.

An alert was sent out at 2:17 a.m. reporting that all eastbound lanes had been closed. Two lanes were reopened by 6 a.m. as the morning commute began to build, but traffic on the busy freeway was backed up through all the San Francisco exits.

By 6:40 a.m., all lanes were reopened. The CHP was still warning morning commuters to be ready for slow traffic and recommended commuters heading into San Francisco to take 280.