The investigation into the disappearance of a Northern California nursing student is reportedly focusing on a former female friend of the missing woman.  

Authorities have issued search warrants and interviewed 15 to 20 relatives and acquaintances as they search for 26-year-old Michelle Le who disappeared six days ago during a break from a clinical rotation. Some of those questioned are considered persons of interest, but no arrests have been made.

KGO-TV reported Thursday that Le's former friend, Giselle Estaban, is among those considered a person of interest in the case.

Estaban reportedly attended high school with Le in San Diego. Estaban reportedly claimed Le dated her boyfriend and then ruined her relationship with the man, who is also the father of her 5-year-old child, according to the station. 

Police reportedly swarmed Estaban's apartment and confiscated her cellphone and computer after executing a search warrant.

"We know she's a former friend of Michelle's ... They've been disgruntled with each other in the past," Hayward Police Lt. Roger Keener told the station. 

"Currently, she's the only one we felt we had enough to pursue further," Keener said. "It doesn't mean that more aren't going to come to light."

Estaban has denied any involvement in Le's disappearance and no charges have been filed against her. 

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 cellphone 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.

Anyone with information on Le's whereabouts is being urged to call the Hayward Police Department at 510-293-7000.

The Associated Press contributed to this report