Obama attended a memorial service at Baylor University for those killed in last week's explosion in nearby West, Texas, even as crews continued to search for answers to what caused the blast or whether foul play was involved.
The April 17 explosion left a crater more than 90 feet wide and damaged dozens of buildings, displacing many residents from their homes. The Insurance Council of Texas estimates it caused more than $100 million in damage.
The blast came minutes after a fire was reported at the West Fertilizer plant, operated by Adair Grain Inc. Ten of those killed were first responders who sped out to the nighttime blaze.
The memorial service honored those first responders and two civilians who tried to fight the fire and were posthumously named volunteer first responders. Among the dead were brothers Douglas and Robert Snokhous, West High School graduates who volunteered together for the town's fire department.
The service opened with a photo slide show set to country music that was projected onto a movie screen over their 12 flag-draped coffins. It showed images of the men from their childhood, their weddings and other moments throughout lives filled with children and friends. Mourners were given programs with full-page profiles of each of the victims, describing their lives, their values and their faith.
Texas Gov. Rick Perry eulogized the unpaid firefighters and first responders, lamenting that each had a personal story and journey that drew to a close too soon.
"These are volunteers. Ordinary individuals blessed with extraordinary courage and a determination to do what they could to save lives," he said. "They're the ones who proudly said 'not on my watch' in the moments immediately following that explosion."
Perry's remarks were followed by video of the victims' grim-faced family members remembering their lives and expressing pride for their heroism. The brother-in-law of Cody Dragoo, another volunteer firefighter, remembered how Dragoo would leave notes for his wife, Patty, when he was traveling, and how he loved hunting and NASCAR.
Obama added his appearance at the memorial service onto a long-planned trip to Texas for Thursday's opening of George W. Bush's presidential library at Southern Methodist University. Bush sent his sympathies in a statement read at the service by Baylor President Ken Starr, the former special prosecutor who investigated President Bill Clinton.
Obama's solemn reflections at the memorial required an abrupt shift in tone by the president, whose morning in Dallas was filled with smiles, music and pageantry as he and the other four living presidents celebrated one of their own. Less than an hour later, Obama was airborne over West, circling the scene of the explosion - still a harrowing site more than a week after tragedy first touched the small Texas town.
From his helicopter, Obama saw what looked like a massive construction site, with cranes and dozens of vehicles dotting a wide swath of brown earth. Piles of burnt rubble and scorched earth were clearly visible. From his helicopter, Obama could also see the school field first responder used as a staging ground.
Obama has made such a trip countless times before, touring damage and consoling survivors of other disasters including Hurricane Irene, Hurricane Sandy and a string of mass shootings. It was just one week ago that Obama was in Boston, offering solace to the nation at a memorial for victims of the Boston Marathon bombing, another larger-than-life tragedy that compounded the nation's grief the same week as the explosion in West.
A parade of fire trucks and other first responders' vehicles paraded through Waco en route to the ceremony at Baylor's sports arena. The vehicles entered under an archway formed by the ladders from two fire trucks with an American flag hung between them.
Nearly 10,000 packed the Ferrell Center, more than three times West's entire population of 2,700. Many of the mourners wore the uniforms of police, firefighters and paramedics and wiped tears from their eyes.
Brian Crawford, fire chief in the Dallas suburb of Plano, attended with 11 others from his department even though they live 100 miles from West.
"With these unfortunate circumstances, it's time to show we are all a family," Crawford said. "These were our brothers and they paid the price."
After the service, the president and first lady Michelle Obama were planning to visit privately with relatives and friends of firefighters killed in the explosion, the White House said.