According to several sources, it was in fact his kirpan. It is NPR and the Associated Press who reported it was a butter knife....probably in ignorance.
Satwant Singh approached him in the lobby and tried to stop him from hurting others and disrespecting Guru Sahib. From what we understand, he attempted to use a kirpan or talwar to attack and tackle the shooter but was shot in the back after a struggle. This is the correct use of a kirpan, protecting the innocent and those that are unable to protect themselves.