The suspects are Anna Nina Jason and Steve puzzle

A man was found murdered in his office.
The suspects are Anna, Nina, Jason and Steve.
The only clue is a set of numbers written on a calendar by the victim with his blood.
The numbers are, 1, 4, 9, 10, 11.
Who is the killer?

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1 - January
4 - April
9 - September
10 - October
11 - November
first letter of months.

JASON is the killer