(CNN) -- "I love you Luke!!"
That's how Ryan Bresette closed a Facebook post on Saturday, a day after his Kansas family -- fresh off a beachfront vacation -- experienced the unthinkable when a display board crashed down on them inside Alabama's Birmingham-Shuttlesworth International Airport.
The crushing weight of the digital sign killed Luke Bresette and left his mother, Heather, in critical condition at a Birmingham hospital, officials said.
"Words cannot describe the pain we feel," Ryan Bresette wrote Saturday on Facebook, urging people attending church to wear sports jerseys to services in tribute to his sports-loving son.
"Thank you all for the love and support. Keep praying and praying and praying."
Posts on what appears to be Anna Bresette's Twitter page said that her mother, Heather, broke both her ankles and her left pelvis; her brother Tyler suffered a concussion; and another brother, Sam, broke his left leg and nose. Justin Cohen, from Children's of Alabama hospital, told CNN on Saturday night that Tyler and Sam Bresette were in fair condition.
"Seeing all these little kids in the hospital breaks my heart," Anna Bressette wrote.
What caused the large digital sign -- which showed when and where flights were arriving to and departing -- to collapse around 1:30 p.m. (2:30 p.m. ET) Friday?
On Saturday, authorities still didn't know.
"(Birmingham Airport Authority) President and CEO Al Denson and the staff are continuing to work with officials to investigate," said Gaynell Hendricks, the chairwoman of the authority's board. "This includes discussions with contractors, subcontractors that are responsible for the modernization of the ... airport."
The incident happened in a new part of the airport, in a public area before security checkpoints. According to CNN affiliate WVTM, the area opened up March 13 and includes a covered parking deck, children's play area, eating establishments, a new baggage system and other features.
Larry Snyder was riding up the escalator when he saw the display board fall, telling CNN affiliate WIAT that one family was especially affected.
"The family was crushed, little kids crushed underneath the sign. And everybody was scattering to lift it up," Snyder said, adding that he was among those who helped.
Video footage showed people sweeping up the area and a cut-out spot along a wall with protruding wires, where the sign once stood.
Whatever the reason, it was clear the Overland Park, Kansas, family was in the wrong place at the wrong time.
Birmingham Mayor William Bell said he'd met with Luke's father, describing him as a "heartbroken" man who "wants and deserves answers."
"I cannot come close to understanding his heartache," the mayor said in a statement. "But I will get answers."
Luke Bresette was a fifth grader at St. Thomas More, a Catholic school in Kansas City, Missouri, according to CNN affiliate KCTV. Father Don Farnan, a priest of the affiliated parish, described him as a "normal kid" who'd been on the city champion basketball team and was always at Mass on Sunday mornings.
"He lived a good moral life," Farnan told the Kansas City station. "He was shaped with the behaviors and the attitudes and the strength of what helps make society a good one."
In the days prior, Ryan Bresette had put up several messages and photos on Facebook about the family's vacation in Destin, Florida -- including several showing two of his sons playing in the Gulf water, with no one else in sight.
His post Saturday, by contrast, reflected his grief and affection for his late son.
"I miss and love Luke so very much."
CNN's Janet DiGiacomo contributed to this report.