I'd guesstimate Mexican Mafia hit squad.
5 seconds sound clip from the Kill Bill: Volume 1 movie soundboard.
You can hear this line at 00:18:13 in the Blu-ray version of the movie.
Quote context
[...]
- What'd I tell you, Pop? It's like a goddamn Nicaraguan death squad.
- You better shit-can that blasphemy, boy. You're in a house of worship.
- Well, this is definitely the work of professionals.
- I'd guesstimate Mexican Mafia hit squad.
- Well, a sure and steady hand did this.
- This ain't no squirrelly amateur. This is the work of a salty dog.
- You can tell by the cleanliness of the carnage.
[...]