One of the nation's biggest July 4th firework displays turned into a 15-second bust after a "premature ignition" caused pyrotechnics spread across five barges in the San Diego Bay to go off all at once.

Thousands of onlookers had spent their July 4 th camped out along San Diego's beaches and parks Wednesday to catch a glimpse of the city's 12 th annual Big Bay Boom fireworks display, billed by organizers this year as being "bigger and more intense than in past years."

The big show was expected to start at 9 p.m. and last 18 minutes but at 8:55 p.m., onlookers heard an explosion of booms and looked up to see what appeared to be a fire in the sky, but was actually the fireworks, just not in the breathtaking, coordinated display that organizers had spent months planning.

After 15 seconds the music continued, but the light show ended. The abrupt start and stop left people wondering if that was really what they had waited hours to see.

"The fireworks went off all at once. Was that it?," @JenBoydSD wrote on Twitter.

"Ooooo ahhhh. Said no one in San Diego bay #fail #bigbayboom," wrote @teaganlyndy.

Nearly 25 minutes later, around 9:20 p.m., when the show was expected to end, the stunned onlookers got their answer. That 15-second display was it, organizers said. There would be no re-do.

The Port of San Diego took to Twitter immediately after the explosion to apologize for what it called a "technical glitch" and said event producers were investigating the cause.

In a statement issued nearly three hours later, just before midnight, the port said that the mistake was caused by technicians who sent a signal "to the barges that would set the timing for the rest of the show after the introduction."

Garden State Fireworks, the firm hired by the city to produce the show, acknowledged the mistake. "We will be working throughout the night to determine what technical problem caused the entire show to be launched in about 15 seconds. We apologize for the brevity of the show and the technical difficulties," the company said in a statement posted on www.bigbayboom.com.

No injuries were reported but the force of the fireworks exploding at once was so great that a large rumble could reportedly be felt throughout downtown San Diego just before 9 p.m.

The Associated Press contributed to this report.