Air carrier flight crew reported an NMAC during departure climb when another aircraft apparently departed their cleared altitude prematurely.
Synopsis
Air carrier flight crew reported an NMAC during departure climb when another aircraft apparently departed their cleared altitude prematurely.
Narrative
We were climbing out on the ZZZZZ departure; tower handed us off to departure. When we checked on with departure; we were given traffic advisory; company traffic at 2 o'clock level at 9;000 feet. We acknowledge and said traffic in sight. We got 'traffic' on the TCAS; so I begin to decrease my climb rate to less than 1;000 feet per minute. At 7;900 feet we got 'TCAS RA DESCEND'; I complied with the 'TCAS RA DESCEND' and through 7;800 feet we got clear of conflict. I looked out and saw an aircraft passing right to left; less than 500 feet at our altitude. We questioned ATC; to make sure we heard that they were supposed to have been level at 9;000 feet; and he said they we're supposed to be level at 9;000; and then he said I guess they started down early. Didn't quite know what that meant; but this was definitely a near miss.ATC should pay closer attention when you've got converging traffic in a situation like this; which could result in a serious accident or incident. The other aircraft definitely was not at their assigned altitude; and ATC should have gotten some warning that they were descending. This was a real event; and shouldn't be taken lightly. I know we are human and make mistakes; but there have to be some safeguards.
Second reporter narrative
[Report narrative contained no additional information]
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.