Drop tablespace including contents and datafiles removes 2 of 3 datafiles from ASM

Executed alter tablespace <> offline;  followed by drop tablespace <> including contents and datafiles commands via sqlplus.  No error was noted during execution, feedback indicated tablespace was dropped successfully, but an item in the alert log noted error ORA-1265 unable to delete file .... followed by ORA-15028 file in use.  This error occurred for 1 file only.  The other 2 files were dropped from ASM.  Since the tablespace was taken offline first, I presumed the file was being accessed by the backup.

What step(s) need to be followed to delete the file from ASM? 

Oracle Release 11.2.0.3 on Linux

thank you.