(Reuters) - Four cases have been added to a list of shootings in Phoenix, Arizona, by a serial shooter or shooters who may now be responsible for at least seven homicides since March, police said on Tuesday.

The fatal shooting of a 55-year-old woman in April, the wounding of a 16-year-old youth and a 21-year-old man in separate incidents in March, and the shooting of an unoccupied vehicle in June were linked to a string of shootings in the Phoenix metro area, the Phoenix Police Department said on its Facebook page.

"Someone out there knows who did this," said Phoenix Police Chief Joseph Yahner, who called on the community to help authorities solve the crimes.

The same suspect or suspects are also believed to be responsible for fatally shooting three women in front of a home and the killing of two men in separate incidents in June, and the fatal shooting of a man in April, police said.

Many of the shootings have occurred in west Phoenix and all of the incidents have occurred at night, according to police.

(Reporting by Brendan O'Brien in Milwaukee; Editing by Paul Tait)