Concord Police were called by a Waltham social worker at 3:30 a.m. on Tuesday about a 40-year-old woman who was missing from her home.

Lt. Roy Mulcahy said the caller suspected the woman may have been suicidal, and may be at Walden Pond.

Mulcahy said he notified the Waltham PD to search the woman's home, but she was not there. The Concord PD searched for her car and found it at Arena Terrace.

Mulcahy said the same woman was involved in a search and rescue at Walden Pond last year. He said a family member told police that if her car was in Concord, she was probably in the woods at Walden Pond.

At that point, Mulcahy said the police contacted the State Police's Concord barracks for assistance. Additionally, the Concord K-9 unit, Sharik the dog, and Officers Sylvia Toumayan and Chris Troiano began tracking the woman from Arena Terrace.

"They made their way in from the railroad tracks," said Mulcahy.

There were K-9 units from Wakefield and Billerica standing by, but not deployed.

"You use everything you have in a case like this," said Mulcahy. "You can't take chances."

Mulcahy said at about 6 a.m. the state police helicopter that had been flying low over the state park located the woman on the far west side of the pond.

"At 6:05 a.m. the helicopter was on the scene and flashed with an infrared beam that a party was found," said state trooper Sean Lewis. "He stayed above her and ground crews were able to drop a blanket to her."

With the helicopter over the woman, Toumayan, Troiano and Sharik, the police dog, joined EMTs from Concord to reach the woman who was desribed as "semi-conscious." Mulcahy said her clothes were "soaking wet."

The EMTs treated her at the scene and transported her to Emerson Hospital, Mulcahy said.