An unmanned SpaceX Falcon 9 rocket bound for the International Space Station exploded a couple of minutes after liftoff Sunday morning. It was the third cargo mission to the space station to be lost in recent months.

Elon Musk, SpaceX's founder tweeted that "there was an overpressure event in the upper stage liquid oxygen tank." He added: "That's all we can say with confidence right now. Will have more to say following a thorough" analysis.

NASA officials said it was not clear what caused the explosion. During an afternoon press conference William Gerstenmaier, NASA's associate administrator for human exploration and operations, said there was "no negligence here."

The three failures from three different launch providers show "the challenges facing engineering and the challenges facing space flight in general."

The rocket took off from Cape Canaveral, Fla., at 10:21 a.m., and everything seemed fine until 2 minutes at 19 seconds. Then video of the launch showed harrowing, if now familiar, images of a rocket exploding into a plume of smoke. The Falcon 9 was carrying more than 4,000 pounds of food and supplies to the space station, where American Scott Kelly is spending a year. There were no astronauts onboard.

The explosion also lost many student experiments and a water filtration system. Also onboard was a piece of hardware that would be used to help two new crew vehicles dock to the station.

The failure follows two earlier mishaps, which had put tremendous pressure on SpaceX to deliver a successful flight. An Orbital Antares rocket blew up in October, and then a Russian Progress 59 spun out of control after reaching orbit. The Orbital failure raised questions about NASA's bold plan to outsource the cargo resupply mission to contractors in the wake of the space shuttle retirement in 2011.

“We are disappointed in the loss of the latest SpaceX cargo resupply mission to the International Space Station," NASA Administrator Charles Bolden said in a statement. "However, the astronauts are safe aboard the station and have sufficient supplies for the next several months. We will work closely with SpaceX to understand what happened, fix the problem and return to flight. The commercial cargo program was designed to accommodate loss of cargo vehicles."

But before the launch, Stephanie Schierholz, a NASA spokeswoman, said that the station had plenty of supplies on board and that the crew would be fine even if there was another failure.

“They supply the station with all these contingencies in mind,” Schierholz said.

A NASA slide from an April presentation said that with current food levels, the space station would reach what NASA calls “reserve level” on July 24 and run out by Sept. 5, according to SpaceNews.

Schierholz said, however, that the supplies would last until the fall, although she could not provide a precise date. Even if something were to go wrong with the SpaceX flight, she said, there are eight more scheduled this year, including several this summer, “so there are plenty of ways to ensure the station continues to be well-supplied.”

On July 3, the Russians are scheduled to fly another Progress 59 to the station before three more crew members arrive later in the month. Then in August, a Japanese HTV-5 is scheduled to send more supplies, followed by another SpaceX launch in September. Despite its failure, Orbital ATK is expected to launch another resupply mission later this year.

But with the explosion, both of the contractors NASA relies on to get critical food and supplies to the space station have now had explosions within eight months of each other.

After the retirement of the space shuttle, NASA lost its ability to fly astronauts from U.S. soil, and has been paying the Russians more than $70 million a seat to fly American astronauts to the station. But NASA hopes to use contractors to end that dependence, and last year awarded contracts to SpaceX and Boeing to develop a capsules that can carry astronauts to the space station.

That is supposed to happen by 2017, but Congress recently slashed more than $300 million from the program, which Sen. Bill Nelson (D-Fla.) said would delay the mission by two years. It’s not clear what impact SpaceX’s catastrophic failure on Sunday will have on that program.

Gerstenmaier said that he did not believe the explosion would affect the timeline. He said NASA and SpaceX could learn from the experience and apply the knowledge to its program to fly astronauts to space: "It could help us to nail down designs and move forward."

Still, he said: “This was a blow to us. We lost a lot of important research equipment on this flight.”

The explosion will raise questions about NASA's bold plan to rely so heavily on contractors, even though SpaceX had a track record of six successful official missions to the station and one test flight going into Sunday.

Bolden defended SpaceX Sunday, saying it "has demonstrated extraordinary capabilities in its first six cargo resupply missions to the station, and we know they can replicate that success. We will work with and support SpaceX to assess what happened, understand the specifics of the failure and correct it to move forward.”

Eric Stallmer, the president of the Commercial Spaceflight Federation said in an interview Sunday, "this isn’t the first time [there's been a failure]; it won’t be the last time. I think we need to be patient, conduct the investigation and get back flying. I don’t think we have a choice."

"I’m confident in SpaceX that they’ll figure out this problem and get back to flying," he said.

Still, Stallmer added, "days like this stink."

During the afternoon press conference, Gwynne Shotwell, SpaceX's president, said the company would be dedicated to "find out what happened, to fix what happened and get back to flight. ... It's certainly time to pause to make sure we're doing everything we need to do."

Instead of trying to figure out why their rocket exploded, SpaceX had hoped to be celebrating the successful landing of the rocket's booster on a floating platform at sea.

Typically, rocket boosters are used once, burning up or crashing into the ocean after liftoff. But Musk, the billionaire co-founder of PayPal and Tesla, has been working on creating reusable rockets that act like airplanes—fly, land, then fly again. If SpaceX can repeatedly land rockets and reuse them, it would mark a significant step toward making spaceflight much more affordable.

But for now, its focus is on the explosion.

"What happened today is why I hold my breath and say a little prayer every time we launch a big rocket," Nelson said in a tweet.