https://en.wikipedia.org/wiki/Murder_(United_States_law)#Degrees

First-degree murder: any intentional murder that is willful and premeditated with malice aforethought. Felony murder, a charge that may be filed against a defendant who is involved in a dangerous crime where a death results from the crime,is typically first-degree.

a conspiracy is an agreement between two or more persons to commit a crime at some time in the future.

For example when Charles "Charlie the Hat" Stango was find and preparing the olivieri murder this a conspiracy to commit murder,if the hypotethycal shooter of olivieri would be caught this ould be a first degree murder.