The shooting happened early Sunday morning outside a home in Westchester.
Thirty-one-year-old Brian Himber is charged with first degree murder.
The victim is identified as 29-year old Tracy Mays.
One of the party guests tells ABC7 they were a couple attending the party.
"They had a confrontation in the front of the house - they were arguing," said Kim Kane. "And then some of the boys, the older boys were looking out the window and saw him shoot her."
The witness says Himber then went into the garage and shot himself.
He is hospitalized in critical condition.
State Police say Himber has been relieved of his powers and remains in custody.
No service weapon was used.