Cno tower controller reported an aircraft entering their pattern reported a near midair collision.

Attributes
ACN	1644359
Time
Date	201905
Local Time Of Day	1201-1800
Place
Locale Reference	CNO.Airport
State Reference	CA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Small Aircraft
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	None
Flight Plan	VFR
Aircraft 2
Make Model Name	Small Aircraft
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 6
Events
Anomaly	ATC Issue All Types Conflict NMAC
Miss Distance	Vertical 500

Narrative:

Aircraft X checked in approximately 12 miles southeast for landing at 025 feet. Aircraft X was issued to enter left base; east of the wash approximately 2 minutes later aircraft X reported a near miss with an aircraft Y. Aircraft X stated the aircraft passed behind about 500 feet lower. A target was observed with no altitude indication passing behind aircraft X speed indicate 140 knots. Target was not observed prior to the report due to other aircraft in the vicinity of the airport. I should better scan for fast moving aircraft departing adjacent airports.

Original NASA ASRS Text

Title: CNO Tower Controller reported an aircraft entering their pattern reported a NMAC.

Narrative: Aircraft X checked in approximately 12 miles southeast for landing at 025 feet. Aircraft X was issued to enter left base; east of the wash approximately 2 minutes later Aircraft X reported a near miss with an Aircraft Y. Aircraft X stated the aircraft passed behind about 500 feet lower. A target was observed with no altitude indication passing behind Aircraft X speed indicate 140 knots. Target was not observed prior to the report due to other aircraft in the vicinity of the airport. I should better scan for fast moving aircraft departing adjacent airports.

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.