Chester County community members were grieving Monday after the weekend deaths of two Downingtown siblings who were killed when an allegedly drunk driver slammed into their family's minivan.

The teenagers - Miles and Charlotte Hannagan - were ejected from their seats when a pickup truck struck the minivan in Exton just before 7 p.m. Saturday, police said. Each had been wearing seat belts at the time.

Miles, 19, was a second-year student at West Chester University. Charlotte, 18, was a junior at Downingtown High School West.

Thomas Muir, 25, of Media, faces vehicular-homicide and other charges in connection with the crash at the intersection of Route 100 and North Ship Road. Police say he was drunk and driving his GMC Sierra pickup truck at a high speed when it struck the van.

Also injured in the crash were the teens' father, Paul Hannagan, who was driving the Honda Odyssey; their mother; and two other adult relatives, police said. Each was taken to Paoli Hospital for treatment.

At West Chester, Miles Hannagan had been studying chemistry and biology, said Pam Sheridan, executive director of the university's office of communications. He lived in Downingtown and commuted to school, she said.

Downingtown High School West, which was closed Monday for Presidents' Day, said it would make grief counselors available for students.

"Words cannot express the sadness that our school community is feeling at this time," school officials said in a statement.

As the news spread, classmates and friends posted remembrances on social media.

"Miles Hannagan was one of the smartest, most dedicated bio majors I've met since I've been at WCU. #RIP," Twitter user Sabrina Aegbuniwe said Monday.

"RIP Charlotte Hannagan. One of the most talented people I've ever met," tweeted user @court-diec.

Uwchlan Township Police Chief Joseph Pontarelli called the deaths "an absolutely tragic loss of life."

Witnesses told investigators that the pickup truck was driven in an erratic manner at a high speed before the accident.

Officers who responded to the scene noted that Muir smelled of alcohol and failed field sobriety tests, authorities said.

His blood-alcohol level was checked at the Chester County Hospital, police said, though officials did not disclose those results.

In March 2014, Muir was arrested in Chester County and charged with use and possession of drugs, records show. He was accepted into the Chester County Drug Court Program. In April 2011, he was arrested in Delaware County and pleaded guilty to disorderly conduct and public drunkenness, according to court records.

Muir was being held in the Chester County Prison on $75,000 bail.

A preliminary hearing is scheduled for Feb. 24.

jmcdaniel@phillynews.com

610-313-8205

@McDanielJustine

Inquirer staff writer Mari Schaefer contributed to this article.