Authorities have issued search warrants and interviewed 15 to 20 relatives and acquaintances as they search for a Northern California nursing student who went missing five days ago during a break from a clinical rotation, police said Wednesday.

Some of those questioned in the disappearance of 26-year-old Michelle Le were considered persons of interest, but no arrests have been made, Hayward police Lt. Roger Keener said.

One person was brought to the police station for questioning and later released. Keener did not identify the person.

"Our overarching goal here is to find Michelle and to find out why she's missing," Keener said, noting that former boyfriends were among those interviewed.

Le was last seen Friday evening heading to a parking garage at Kaiser Hospital in Hayward. Her locked Honda SUV was later found a few blocks away.

Investigators were examining footage from the parking structure's security cameras. Police believe Le had her cell phone with her when she disappeared, but calls to the number have gone unanswered.

Michelle Le's father traveled from Vietnam to assist in the search. The family has offered a $20,000 reward for information leading to her safe return.

Keener played down a possible connection to another case that remains unsolved in a city about 55 miles from the spot where Le disappeared.

Authorities in Fairfield are investigating the April 25, 2010, disappearance of nursing school graduate Bichphuong "Phuong" Le, who was 24 at the time and no relation to Michelle Le.

Phuong Le's body was found in rural Napa County 12 days after she disappeared outside a Fairfield bookstore. Her unlocked car was left in the parking lot.

Despite the similarities between the two cases — including the same last name and the nursing connection — Keener said the information available does not point to a direct connection.

"While we can't eliminate it 100 percent, we are probably not going to focus on that direction," he said.