HARRIS COUNTY -

The search for a missing pregnant woman entered its fifth day on Tuesday.

Authorities and volunteers with Texas EquuSearch are searching for Melissa Sowders, 26, who disappeared on Thursday after she was supposed to meet her ex-husband, Matthew Sowders.

Investigators suspect foul play in her disappearance.

Search crews were back out by the Cypress Creek located along the North Freeway and FM 1960 Tuesday morning.

Crews say they are dealing with cold temperatures and lots of tough terrain, but they say they won’t stop until they find Sowders.

Her car was found over the weekend a short distance away from the fast food restaurant where she was supposed to meet her ex-husband. That’s the same area where crews from are looking for her.

After several days of searching for the young mother of four, crews are calling the search a recovery mission.

 “It's a tough pill to swallow, but you know, I think the most we can hope for is to get her located as quick as possible,” said Tim Miller with Texas EquuSearch.

On Monday, a sonar device spotted something underwater but volunteers ran into problems with equipment malfunctioning in the tough terrain.

“We just got some treacherous things out here in the water,” said Miller. “I mean, there's a lot of concrete.”

Divers were called to the creek to search through the murky waters, hoping to find the missing mom.

Family members say that Melissa’s ex-husband liked to fish right here in this creek. They believe that she may be nearby. They also say she was concerned about meeting with him because of her safety.

According to Melissa’s family, the four children live with their father.

Sowder's ex-husband is no longer cooperating with the investigation as of Tuesday. Investigators have taken the ex-husband's car into evidence or processing.

Harris County Homicide detectives are not releasing any new details.