The incidents occurred shortly before midnight in the 1300 block of West Seventh Street on San Bernardino's Westside.
Officers were investigating the non-fatal stabbing of a 29-year-old man when shots were fired.
Investigators found that Felipe Jason Moreno, 20, of San Bernardino had been fatally shot, while Daniel Gomez, his 22-year-old cousin, had been wounded.
Moreno was the city's 47th homicide victim this year, and the first since two homicides on Nov. 26.
In one of those, Robert Forrest Dearmin Jr.
Click points for details | Red = Most recent incident(s)
In the other incident that day, Leon Gregory McLaughlin was fatally shot by an armed robber who had demanded property from his wife at 2020 Guthrie St.
On Friday night, police initially thought that the shootings and stabbing were related to the party, but later concluded it was more "somebody throws a party and people start to come over," said Sgt. Gary Robertson.
"It was a party for family," Elizabeth Chavez said Saturday afternoon, near the intersection of Mount Vernon Avenue and West Seventh.
She said she was at the party and didn't know the men who were shot and stabbed.
The party was winding down when the violence flared up, Chavez said.
Moreno and Gomez were shot in a vehicle as it was driving away from an empty lot east of the house on West Seventh, where the party was being held, Robertson said.
Moreno, who was driving, was shot in the upper torso, and the car came to a rolling stop after he succumbed to his wound. Gomez was shot in the leg, Robertson said.
Shell casings from two calibers were found, which might suggest two shooters, although one set of casings might have been there before Moreno and Gomez were shot, Robertson said.
The stabbing victim sustained a non-life threatening injury, Robertson said.
"He was not totally cooperative" in describing what happened to him, Robertson said.
"We interviewed many people, but nobody provided useful information about suspects," Robertson said.
"We kept hitting dead ends with everyone we talked to," he said.
Several neighbors of the house where the "End of the World" party said Saturday afternoon that they didn't know what had happened in the neighborhood the night before.
As of Saturday afternoon, police had no solid leads on potential suspects, Robertson said.
Anyone with information on these cases is asked to contact police Detective John Munoz at 909-384-5630 or Robertson at 909-841-5361.
Reach Jim via email, call him at 909-386-3855, or find him on Twitter @JSteinbergsRoad.