Dionne Ramirez, 29, who lives about a mile from the refinery, said she had little doubt that elevated pollution harmed her family. Not only have both she and her husband had coughs, but all three of their young sons have suffered from severe chest congestion, sore throats and endless coughing since April. Her 4-year-old had to be hospitalized for two nights because he could not stop coughing, she said.

When the news of the pollution was made public on June 4, Ms. Ramirez was irate. “I didn’t know why they were getting sick or what was going on,” she said. “They are healthy little kids.”

Her experience was echoed by other families living in the shadow of the jumbled smokestacks, pipelines, cylindrical tanks and giant globes of the refinery. Nearly every household on one block of First Avenue, just a half-mile from the BP complex, had someone fall ill during May, residents there said.

“We all became real sick  throwing up, diarrhea, couldn’t keep anything down  and we just thought it was something that was going around,” said Khristina Kelley, who lives with her husband and four children on the street. “But then everybody around here got it.”

Ms. Kelley said the release of chemicals was less troubling to her than the company’s silence. “I’m worried that one day I’ll take my kids to the doctor and something that could have been prevented wasn’t prevented because we didn’t know to the last moment,” she said.

Officials in Texas City said they were not informed of the scale of the release until it was over. BP said it met the requirements of state law by informing state officials of the release in writing on April 7, then filing a final report on June 4, after the equipment was fixed.

That final report said the release of chemicals had gone on for 959 hours, until May 16. Among other pollutants, the plant had released 17,000 pounds of benzene; 37,000 pounds of nitrogen oxides, which can cause respiratory problems; and 186,000 pounds of carbon monoxide. Another 262,000 pounds of various volatile organic compounds also escaped.