TORONTO - Police are calling the discovery of a backpack belonging to a teenager who mysteriously vanished heading into school almost four weeks ago an important find.

Mariam Makhniashvili's new-looking backpack, some of its contents strewn about, was found in a rear parking lot of an apartment complex a few minutes drive from her school, police said Friday.

"This is the first tangible item that we've discovered," Det.-Sgt. Dan Nealon said as he stood next to a poster of the black Hercules brand backpack.

"We're always hopeful that something like this will lead to the whereabouts of Mariam."

Makhniashvili was last seen on the morning of Sept. 14, when she left her younger brother's side outside Forest Hill Collegiate as they headed into the high school.

Despite public appeals from police and the teen's family, publication of photographs and video, there's been no indication of what became of her.

Still, police expressed optimism.

"We're not stymied," Nealon said.

"We have other places that we can go. We're, of course, optimistic and hopeful."

The bag, found by a man under some trees in the parking lot on Thursday, was positively identified as the teen's by books inside. Some of its contents were strewn about as if someone had gone through it and discarded some items.

Police said they were "going down every avenue" the discovery could possibly lead to.

In following up on the find, police said they learned someone else might have seen the bag in the vicinity two weeks ago and moved it, but didn't realize its significance at the time.

The parents were "quite shaken up" by the backpack find, Nealon said.

"Like any parent who would feel being that much closer to finding their daughter, certainly is something that I couldn't imagine what they're feeling; they're upset."

Vakhtang Makhniashvili planned to talk about the disappearance of his daughter later Friday.

About 40 uniformed officers on Friday were canvassing the area where the bag was found and testing it for any forensic evidence.

Two weeks ago, police announced they were expanding their search nationally and internationally. Police also searched a park in the area of the school, but came up empty handed.

Makhniashvili had been in the country for three months when she disappeared.

She arrived in Toronto from the Republic of Georgia in June to join her parents, who had been living in California.

Police and parents insist there were no family problems, and said they considered it a missing person's case.