San Francisco police are downplaying suggestions that the devil-worshiping loner known as the “Night Stalker” might have had a child sidekick when he raped and strangled a San Francisco girl more than 30 years ago.

A man who was linked by DNA evidence to a San Francisco basement where the 9-year-old girl was slain by Richard Ramirez at the outset of a serial-killing spree through California was probably nowhere near the city when the girl died, officials said.

Authorities revealed this week that this second person was a juvenile at the time of the killing in 1984 — and that investigators had looked into his possible role. But they say he has not been connected to the killing.

The girl, Mei “Linda” Leung, was found hanging over a pipe below her Tenderloin apartment building at 765 O’Farrell St. on April 10, 1984.

The girl had been with her 8-year-old brother when she lost a dollar bill and went looking for it, police said. The boy wandered away, then came back to the basement and found his sister dead. The case remained unsolved for years — even after the 1985 capture in Los Angeles of Ramirez, who died of cancer in 2013 while on Death Row.

Ramirez, who came to be known as the Night Stalker for a string of Southern California attacks in which he broke into homes and murdered people who had been sleeping, was ultimately convicted of 13 killings, but he is suspected of several other slayings and attacks.

One of those is the rape, stabbing and strangulation death of Mei. In 2009, DNA evidence collected from the basement where she was found was positively matched to Ramirez.

Ramirez’s DNA was discovered on a handkerchief along with a secondary sample, as The Chronicle reported in 2009, leading some to believe the Night Stalker may have worked with an accomplice. Though the second sample was uploaded to an FBI DNA database, it went unidentified for years.

Then, in 2012, the database returned a potential hit, linking the secondary sample to a now-convicted felon, in a development that police did not confirm until this week.

There are some problems with the theory that Ramirez had help, though, officials said.

John Sanchez, crime lab manager for the San Francisco Police Department, said the genetic material from the second person was a much smaller sample than that of Ramirez and never should have been uploaded to the criminal database because incomplete profiles can result in false positives.

“It shouldn’t have gone in because it didn’t meet the standards,” he said. “Any ID (made from the flawed sample) would require further investigation.”

The ensuing investigation didn’t lead police any closer to naming the man as a suspect, said Sgt. Michael Andraychak, a police spokesman.

Investigators interviewed the man, who lived a good distance from San Francisco in April 1984, and there was no evidence he visited the city at or around the time Mei was killed, Andraychak said.

While the man went on to have a felony record, Andraychak said, his most serious offense was weapons possession, and his rap sheet is free of sex crimes.

“We can’t definitively say he wasn’t there,” Andraychak said. “But we interviewed him and we don’t have anything (other than the flawed sample) to connect him to the crime either.”

Mei was slain more than two months before what had been Ramirez’s first known murder, the killing on June 28, 1984, of 79-year-old Jennie Vincow in the Glassell Park area of Los Angeles.

The pace and brutality of his spree — some victims were mutilated, their eyes gouged out — terrorized the state before he was spotted, subdued and pummeled by bystanders in 1985.

Kale Williams is a San Francisco Chronicle staff writer. Email: kwilliams@sfchronicle.com Twitter: @sfkale