HOUSTON — (CNN) The storm has passed, but the full scope of the tragedy is just starting to emerge.

As floodwater slowly recedes in Texas, more bodies surface — many found in vehicles that had been swept away by raging currents.

By Wednesday morning, at least 18 people were found dead in Texas and Oklahoma, as well as 13 in northern Mexico from the same storm system.

But at least 13 people are still missing. And the effort to find them continues to be treacherous.

The country’s fourth-largest city was ravaged by flooding many had not seen in a lifetime. More than 11 inches of rain fell in some parts of Houston just in the past day, on top of relentless rainfall that had already inundated the city.

At least five people died and two remain missing in Houston, authorities there said.

Normally bustling highways turned into rivers. Abandoned cars looked like toys in a bathtub.

“The defining feature of Houston is the small rivers that run through the city,” Mayor Annise Parker said. “Many of them went over their banks and began to flood neighborhoods.”

And the structural loss has been devastating. More than 4,000 properties were severely damaged in Houston, Parker said.

“We’ve seen flooding before, but not nearly to this extreme,” said Gage Mueller, who’s lived in Houston for the past 40 years. “It rains and it rains and it rains, and there’s really nowhere for the water to go. … It’s ridiculous.”

In addition to the five dead in Houston, three people died and 11 remain missing in Hays County, not far from the Texas capital of Austin. Four other counties reported one death each — Medina, Milam, Travis and Williamson counties.

In Oklahoma, six people died from storm-related injuries. They included a firefighter in Claremore who was trying to perform a water rescue.

A swift-water rescue attempt turned perilous when the rescue boat capsized in Houston, the mayor’s office said.

An elderly couple who were on the boat remain missing, said Janice Evans, spokeswoman for Houston’s mayor.

The need for rescues was so great that Houston public works trucks were converted into temporary ambulances, responding to 911 calls and pulling several residents out who couldn’t escape their homes.

With any luck, more of the missing will be rescued.

“We have cars littered all over the city,” Parker said, “and as the floodwaters go down, that’s one of the things we’re doing to make sure — that no one was trapped in those vehicles.”

It’s been three days since the McComb family’s vacation cabin was uprooted and washed away by flood currents in Wimberley, Texas.

Jonathan and Laura McComb and their two children were inside. As the cabin floated downstream, it broke apart.

Laura McComb called her sister as the house drifted away.

“A little after 1 o’clock in the morning, she called me and said: ‘I just want you to know the ceiling has caved in, and the house is floating down the water,'” sister Julie Shields told CNN’s Anderson Cooper.

“‘Tell Mom and Dad that I love them. I love you, and pray.'”

Jonathan was eventually found alive, with a collapsed lung and broken sternum. But his wife and two children remain missing.

“We never lose hope,” Jonathan’s father Joe McComb said. “But I think reality is setting in that there is probably a good chance that it might not be the outcome we’re hoping for. But you never give up hope.”

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