A witness told authorities that the driver of a Jeep that struck and killed a 5-month-old boy in a stroller in Loudoun County sped up before the collision and may have been looking at his phone, according to a court document.

At about 8:10 a.m. on Wednesday, the driver of a 2011 Jeep Grand Cherokee was traveling north on Coton Manor Drive and taking a left onto Riverside Parkway when he struck Tristan Schulz and his mother, who were walking north in a crosswalk at that intersection.

The crash killed the child, while his mother, who has not been identified by authorities, was sent to a hospital with injuries that were serious but not life-threatening.

Now, a witness quoted in an affidavit for a search warrant filed in Loudoun Circuit Court said the driver, a 45-year-old Leesburg man, “turned at the last second.”

The witness, who was “traveling directly” behind the Jeep, according to the affidavit, told police he “observed the male driver of the Jeep through the rear window holding a phone in his left hand as if he was watching or reading something” before the crash.

The witness also said the Jeep was “tailgating” the vehicle in front of it, and tried to “beat” the mother and child through the intersection.

“It appeared as if the Jeep was trying to . . . go in front of the female with the stroller, and that the Jeep sped up fast,” the affidavit read.

A spokesman for the Loudoun County Sheriff’s Office said that the case was still under investigation and that no charges had been filed.

Although the affidavit named the driver, who remained on the scene after the crash, The Washington Post is not naming him because he has not been charged with a crime.

The affidavit said Michael P. Kelly, a partner with the Washington law firm Hogan Lovells, identified himself as the man’s attorney at the scene.

Kelly told investigators the man “would be invoking his privilege and would not be providing a statement to investigators,” according to the affidavit.

Reached by phone, Kelly said the man had new counsel, and that attorney declined comment.