New York (CNN) -- From the loudspeakers of New York police patrol cars and search vehicles, Avonte Oquendo's mother calls for him.
She urges the 14-year-old autistic boy to walk toward the flashing blue and white lights of the emergency vehicles that have been combing the streets for him.
The recording is part of the New York Police Department's stepped up search efforts to find Avonte, who was last seen on surveillance video running out of Center Boulevard School in Long Island City on the afternoon of October 4, Keith Brooks, director of operations for CityWide Disaster Services, said Thursday.
"Avonte, this is your mother. You are safe. Walk toward the lights," the message repeats.
The hope is the teen, who is unable to communicate verbally, will hear the sound of his mother's voice and approach the emergency vehicles.
The reward for the safe return of Avonte grew this week to $77,500, as divers joined an ever growing number of police and rescue officers from several states who have been searching for him nonstop for more than a week.
Avonte's mother, Vanessa Fontaine, told CNN's "Piers Morgan Live" that she believes her son is still alive and that someone is probably holding him.
"My message to my son is that I love him and we're going to find him. You'll come home to your family. And for anyone who has him, please be kind and to let him go," she said Thursday night.
NYPD Commissioner Ray Kelly said he is not holding the school safety officer who was on duty when the teen ran out of the Queens high school responsible for the boy's disappearance.
When Avonte approached the front door of his school the day he disappeared, the safety agent told him to go back upstairs, Kelly said at a news conference on Wednesday. Surveillance video then showed Avonte turning and going down another hallway, and exiting the building from a side door, he said.
Surveillance video provided by the police department shows that no supervisor or monitor stopped the 14-year-old when he ran out.
"Even though he's 14, it shouldn't matter. No one should be, you know, allowed to run around the hallways in school. And that security guard should have questioned him," Fontaine said.
Police said Avonte was last seen wearing a gray striped shirt, black jeans and black sneakers. He is 5 feet, 3 inches tall and weighs 125 pounds.
Missing-persons posters are being handed out, and the search has expanded to areas outside New York City, Kelly said.
The teenager is fascinated by trains, his family has said.
Searches of train stations, tracks and tunnels are being conducted at the start of every shift by the Metropolitan Transit Authority, Kelly said, adding that 50 NYPD officers and a task force of detectives are working the case.
Transportation officials in New York suspended overnight track maintenance on the city's transit system this past weekend as workers combed the underground network. All 468 New York City subway stations have been searched, and aviation, harbor and canine officers are mobilized and deployed periodically, the NYPD said.
David Perecman, the Oquendo family's attorney, said it took school officials at least 45 minutes to call police after Avonte left.
"I don't understand how this happens to a special needs child unless there is something in there that failed the family," he told "Piers Morgan Live" on Thursday.
Last week, Perecman said he was looking into how the child was able to leave school grounds unsupervised.
"Right now, we have submitted a Freedom of Information Act request to the city and the various agencies involved, in order to obtain the information for us to know what occurred," he said.
The New York City Department of Education issued a statement saying it is working closely with police. The school is not commenting.
The Oquendo family filed a "notice of claim" on Wednesday, said Perecman, marking the first step of a lawsuit against the city of New York. He declined to give further information about the claim.
Anyone with information about Avonte is asked to contact the NYPD's Crime Stoppers Hotline at 800-577-TIPS. The public can also submit tips at the Crime Stoppers website, nypdcrimestoppers.com or can text to 274637 (CRIMES), then enter TIP577.
CNN's Julia Lull contributed to this report