Updated at 12:50 p.m. Thursday: Revised to include information from the firefighter's arrest warrant affidavit

An 18-year-old woman and her unborn baby were killed early Wednesday in a crash caused by an off-duty Dallas firefighter who is suspected of driving drunk in Cedar Hill, police said.

The firefighter, identified as 45-year-old Horace Shaw III, was arrested and faces a charge of intoxication manslaughter.

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Cedar Hill police and fire crews arrived about 12:45 a.m. in the 200 block of U.S. Highway 67, where a two-door Mercedes-Benz headed south rear-ended a Mazda SUV.

The woman, identified as Alyssa Pimentel of Midlothian, and her boyfriend Isaiah Perez in the SUV were ejected by the impact.

Perez was taken to a hospital with serious injuries.

Horace Shaw III (Dallas County sheriff's department )

The two occupants of the Mercedes were hospitalized with injuries not considered life-threatening.

Alyssa Pimentel (Facebook)

Police believe Shaw was driving fast and was under the influence of alcohol. Investigators were told Shaw had been drinking at a bar before the crash, police said.

Shaw's passenger told police Shaw was "speeding a little bit" and "showing off" before the crash, an arrest warrant affidavit says. A witness told police the car appeared to be going between 100 and 110 mph.

The witness reported seeing sparks and then two cars sliding into the grassy shoulder between Highway 67 and the service road.

Shaw got out of his car, walked to where Pimentel was lying on the ground and told the witness he was a paramedic before lying down as well, police records say.

Shaw faces one count of intoxication manslaughter. He was booked into the Dallas County Jail on Wednesday evening after being released from the hospital, police said.

The city said Shaw has been placed on administrative leave while the department conducts an internal investigation.

Perez suffered a broken leg and fractured neck, he told KXAS-TV (NBC5) and KTVT-TV (Channel 11). After the crash, he ran over to where Pimentel landed.

"She said she loved me but other than that I had seen her fade away," Perez told the station. "It should've been her staying alive and me actually getting hurt."

Perez said he didn't feel anything and it felt like someone hit him in the back of the head.

"I feel like it’s a dream, like I’ll just wake up and she’ll be walking by," Perez said. "If he [the firefighter] was messed up, he should have got a ride from someone else."

Horace Shaw (Dallas Fire-Rescue)

Pimentel's father, Joe Pimentel, told WFAA-TV(Channel 8) that he was devastated. He said the baby boy, who was going to be named Josiah, was due next month.

Alyssa had just picked up Perez from work, and they were on their way home when the crash occurred, he told the station.

Joe described his daughter as a "sweetheart, a kind, caring person who would have done anything for anyone."

Alyssa's mother, Jayne Pimentel, wrote on Facebook that she was disappointed that "an individual who is tasked with saving lives is responsible for taking the lives of our family members."

"As you can imagine, our grief right now is immeasurable," she continued. "We are aware that there is an investigation into the accident and we trust that the authorities will be diligent in seeking justice for Alyssa and Baby Josiah."

The Texas Alcoholic Beverage Commission is assisting in the investigation and will speak with employees at any bars that might have been visited by the driver.

If a licensed business sold alcohol in a manner that contributed to the fatality, the business could face a fine or temporary suspension of their liquor license, public information officer Chris Porter said. Repeated violations could eventually lead to a cancellation of their liquor license.

Staff writer Naheed Rajwani contributed to this report.