HOUSTON -- A year has passed since a massive explosion at the BP plant in Texas City took 15 lives and injured more than 170 people.

BP held a moment of silence at 1:20 p.m. at the refinery.

Victims' families also had their own private memorials.

Relatives and friends visited the gravesite of Lorena Cruz, 32, who was a secretary at the plant.

"We understand more and more how true it is -- here today and gone tomorrow. We had an opportunity to live that and now we have to remember her forever," said Onorio Villareal, Cruz's brother.

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BP still faces criticism for management lapses that may have contributed to the blast. The company also faces a possible Justice Department investigation and is dealing with victims' lawsuits. Some lawsuits have been settled.

The U.S. Chemical Safety and Hazard Investigation Board, one of several agencies investigating the blast, concluded the isomerization unit where the blast occurred had a history of problems. The unit boosts the octane in gasoline.

Faulty sensors didn't warn of gathering vapors, which ignited as the unit was starting up. Several explosions leveled the unit and nearby trailers in which people were working, including James Rowe, 48, and Linda, 47, both of Hornbeck, La.

The CSB's preliminary report in October said part of the unit that relieves pressure was not hooked up to a flare that burns off vapor and could have prevented or minimized the accident.

The report also found that BP fostered bad management at the plant, about 40 miles southeast of Houston, and failed to fix problems. The CSB also found that workers' trailers were too close to the unit. The board asked the U.S. petrochemical industry to rethink the location of trailers in refineries. The CSB's final report is due in the fall.

In September, the U.S. Occupational Safety and Health Administration found BP committed more than 300 willful violations of its rules and fined the company $21.3 million.

BP's own December report blamed failures by management at the refinery, saying it didn't make safety a priority, tolerated risks and failed to communicate. But BP added it "found no evidence of anyone consciously or intentionally taking actions or decisions that put others at risk."

The CSB has credited BP for cooperating with its investigation, making sweeping changes in how it recognizes potential hazards and naming a panel headed by former Secretary of State James A. Baker III to investigate safety and management oversight at U.S. plants.

But CSB Chairwoman Carolyn Merritt said it's up to companies like BP to ensure they properly train employees to prevent accidents.

"In this case and over a long period of time, that was not performed," she said.

Ross Pillari, president of BP Products North America Inc., said the explosion will never be forgotten and that lessons were learned from it.

"As a result of this tragedy we are making significant changes in our approach to process safety and in the way we operate and monitor operations at Texas City and our other manufacturing sites," he said in a statement. "We are striving to attain the highest levels of safety, reliability and environmental performance."

Beyond the statement, BP refused interviews about the explosion.

The refinery processed up to 460,000 barrels of crude oil a day and 3 percent of the nation's gasoline. But it has been shut down since September, when Hurricane Rita threatened the area.

BP has used the time to make improvements, and production was set to resume by the end of the month. BP has said it will invest about $1 billion over the next five years to improve and maintain the site.

BP has installed a new management team at the facility, simplified the organization and improved communication.

The company also removed at least 160 trailers and set up office space for nearly 400 workers at a nearby retail building.

In January, BP set up a shuttle service for use by more than 2,000 workers, reducing traffic, spokesman Scott Dean said. An idling pickup truck near the unit is among four possible sources that triggered the blast.

John Bisney, a spokesman for the American Petroleum Institute, credited BP for an extensive review and analysis of its operation and safety practices.

"There are a lot of challenges in a difficult industry like oil and natural gas," he said. "But we do our best to minimize those risks and provide as safe a working environment as we can."

OSHA spokesman Al Belsky said his agency has referred its investigation to the Justice Department for possible criminal prosecution. Justice officials declined to comment.

"It was a very strong case as far as the referral," Belsky said.

Najm Meshkati, an associate professor of civil and environmental engineering at the University of Southern California, said he believes petrochemical companies like BP have been "giving lip service" to safety.

"But I am very optimistic the industry will make changes to make things safer," said Meshkati, who has researched refinery operations. "What will define the characters of these companies is which ones will take notice of this accident and work proactively."