Missing Christopher Marks was last seen swinging from a tire above the Meramec River by a family member with him Sunday night at the Pacific Palisades Conservation Area, confirmed St. Louis County Police Chief Tim Fitch at the scene Tuesday morning.

Fitch said this effort is still consider a search and rescue endeavor.

Christopher is a 12-year-old from Affton who will be a seventh grader at Rodgers Middle School.

To receive real-time updates about this developing story, please visit Affton-Shrewsbury Patch's Facebook page.

Police are searching specifically up to one mile downstream of where Christopher, his twin sister, stepfather and friends were Sunday evening.

"We will not stop until we've search over and over and over again all the options," said Fitch. "The officers won't rest until they've checked it all many times."

He said the Meramec River is deep at certain spots, and murkiness has made search attempts extremely challenging. "The divers said they couldn't see their hands in front of their faces."

Among the canine units brought in Tuesday are dogs especially trained to pick up scents under the water, Fitch said. "There are multiple search dog teams here, and more enroute today."

Fitch said it was possible that Christopher's body had been swept away, given the depth and currents of the particular part of the river and cove in question. But he also said he still had optimism about the situation because Christopher was a healthy, strong youth.

For more, check out our coverage of Christopher's disappearence.