(CHICAGO) -- The mother of Chicago postal worker Kierra Coles, who mysteriously vanished more than four years ago, is hoping the FBI will take the lead in the search for her daughter.
Coles was about three months pregnant and eager to meet her first child when she disappeared on Oct. 2, 2018, according to her mother, Karen Phillips.
Phillips said every day without her daughter is a mental struggle.
"One minute I want to scream and holler, I want to cry all day," she told ABC News. "But I know I have to keep going, because I still have to live my life and pay my bills. But it's just so stressful. Sometimes I don't even want to get up -- I just want to sleep all day."
Phillips said Wednesday, "I'm hoping the FBI can get us a conviction, or maybe find something that the [Chicago] police hasn't found. ... Working with just the [Chicago] police, it's getting nowhere."
She said she hopes answers will bring her "some type of closure."
Chicago police, who classify the 26-year-old's disappearance as an open and cold case, has not responded to ABC News' request for comment on Phillips wanting the FBI to join the investigation.
The FBI would not comment on this case directly, but a spokesperson said, in general, "The FBI is always willing to work with law enforcement partners to the greatest extent possible to provide critical resources on local cases."
Chicago police released surveillance video in 2022 that captured some of Coles' last known movements on Oct. 2, 2018.
The surveillance video showed a man -- who is considered a person of interest -- arriving at Coles' home, and later Coles and the man were seen driving away in Coles' car.
At about 10:43 p.m. that night, Coles was spotted on surveillance video making ATM withdrawals -- the last known images of her, according to police.
Later that night, Coles' car was parked in another part of the city, police said. The person of interest was seen getting out of the passenger side, but nobody got out of the driver's side, according to police.
The next day, the person of interest was seen parking Coles' car near her home and going inside, police said. He then left Coles' home and drove away in his car.
When the person of interest was interviewed, police said he gave varying accounts of the last time he saw Coles.
As the search continues, Phillips vowed, "I'm never gonna stop looking for my daughter."
"Sometimes I find myself talking to her," she said. "'Kierra, where are you? Give us a sign.'"
ABC News' Sarah Beth Guevara contributed to this report.