Police initially linked nine shooting incidents and seven murders to the so-called "Serial Street Shooter".
Saucedo had been arrested last month in connection with a separate 2015 killing, police said.
Romero, 61, was shot and killed the night of August 16, 2015 near his home in the area of 900 E. Montebello Avenue.
Saucedo pleaded not guilty to that charge.
Four days prior, shots were fired at a home near 900 E. Colter Street, Williams said Monday, but nobody was injured.
Phoenix Police Chief Jeri L. Williams announces, Monday, May 8, 2017, in Phoenix, the arrest of 23-year-old Aaron Juan Saucedo in connection with the serial street shootings that terrorized the Phoenix area over four months in 2016.
It wasn't until March 17, St. Patrick's Day 2016, that the series known as the Serial Street Shootings began.
Police said they investigated whether Saucedo might be the so-called Phoenix freeway shooter believed responsible for nearly 12 incidents, none fatal, on I-10 in 2015.More news: Swansea beats Everton 1-0, out of EPL relegation zone
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But police on Monday said they had tied Saucedo to nine killings in all - eight random victims and one man that he knew. However, they said, none of the evidence between the two series matched.
Police say a man accused of killing two engaged doctors at a luxury Boston condominium building has a criminal history of robbing banks.
Investigators initially believed a serial shooter was responsible for seven deaths a year ago, but in recent weeks they determined two other victims - the one killed in August 2015 and another person slain in January 2016 - were shot by the so-called "serial street shooter" who they believe to be Saucedo.
"Our hearts go out to the surviving families".
"As we turn over our investigation to the Maricopa County Attorney's Office for prosecution, we hope that our community will rest a little easier and that our officers will get a little more sleep knowing that the wheels of justice are finally in motion", Williams said.
The shootings occurred late at night or early in the morning, mostly on the west side of Phoenix.
In the most deadly single attack, two women and a 12-year-old girl were shot to death in front of a home before dawn on June 12, 2016, police said.