The dead baby girl was found in a trash bin in a women’s bathroom at main airport in Phoenix on Oct. 10, 2005. Wrapped in newspapers and a towel, the newborn had been stuffed into a plastic bag from a Marriott Hotel, the police said.

Detectives immediately began investigating the death of the child, who came to be known as Baby Skylar. A medical examiner determined two days after the baby was found that she had been suffocated and was the victim of a homicide. But leads in the case eventually dried up, and the investigation remained dormant for years.

On Tuesday, more than 18 years after the gruesome discovery at Terminal 4 of Phoenix Sky Harbor International Airport, the authorities announced at a news conference that they had identified and arrested the baby’s mother, Annie Anderson, 51, of Washington State, and that she would be charged with first-degree murder in the child’s death.

Ms. Anderson was in custody in Washington on Tuesday, and was awaiting extradition to Maricopa County, Ariz., Lt. James Hester of the Phoenix Police Department said at the news conference.

Among the few and early leads that the police had was the plastic bag in which the baby’s body had been found. That prompted detectives to investigate Marriott hotels in the Phoenix area, Lieutenant Hester said. But those leads and others proved unsuccessful.

Then, in 2019, the Phoenix police partnered with the F.B.I. to use genetic genealogy, an emerging tool in solving cold cases, to look into the Baby Skylar mystery, Lieutenant Hester said.