Narrative:

Norcal approach cleared us to descend via the SLMMR4 except maintain 11;000 ft. Descending out of 12;000 ft. We began monitoring the approaching traffic closely. The target was climbing and heading straight toward us. As we began leveling off at 11;000 ft. The traffic was continuing to climb and showing 500 ft. Below us. Norcal gave us a left turn 15 degrees for the traffic. They we're showing -300 ft. At this time on our TCAS. As we started the turn we immediately received a traffic advisory followed by a traffic resolution. I clicked off the autopilot and climbed as instructed by the TR. [The captain] notified ATC that we are responding to a TR. ATC never acknowledged and gave us new instructions to descend to 6;000 ft. And heading 245. All that was said was traffic no factor. We never saw the traffic visually.norcal is typically handling several aircraft simultaneously. We were given the descent via except maintain 11;000 ft. For a reason. We assumed it was for traffic; but was never stated. ATC panicked when they realized what was going on and never acknowledged that there was an aircraft that potentially was on a collision course with us. Mistakes happen quite often; but given the crossing restriction without any other narrative or further communication until seconds before the TR is unacceptable. Possibly ATC was preoccupied with other aircraft that distracted them from our conflict.

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Original NASA ASRS Text

Title: Air Carrier pilot reported an NMAC and a Traffic Alert during descent.

Narrative: NorCal Approach cleared us to descend via the SLMMR4 except maintain 11;000 ft. Descending out of 12;000 ft. we began monitoring the approaching traffic closely. The target was climbing and heading straight toward us. As we began leveling off at 11;000 ft. the traffic was continuing to climb and showing 500 ft. below us. NorCal gave us a left turn 15 degrees for the traffic. They we're showing -300 ft. at this time on our TCAS. As we started the turn we immediately received a Traffic Advisory followed by a Traffic Resolution. I clicked off the autopilot and climbed as instructed by the TR. [The Captain] notified ATC that we are responding to a TR. ATC never acknowledged and gave us new instructions to descend to 6;000 ft. and heading 245. All that was said was traffic no factor. We never saw the traffic visually.NorCal is typically handling several aircraft simultaneously. We were given the descent via except maintain 11;000 ft. for a reason. We assumed it was for traffic; but was never stated. ATC panicked when they realized what was going on and never acknowledged that there was an aircraft that potentially was on a collision course with us. Mistakes happen quite often; but given the crossing restriction without any other narrative or further communication until seconds before the TR is unacceptable. Possibly ATC was preoccupied with other aircraft that distracted them from our conflict.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.