State inspectors visited Tonawanda Coke at least once a year and, until recently, never cited the company for a small bleeder valve that spewed coke-oven gas with toxic benzene.

That same valve is at the center of a criminal prosecution charging the manufacturer with polluting the air around its Town of Tonawanda plant.

How did the state miss it for so long?

Lawyers for Tonawanda Coke are asking that question as part of a defense that suggests state and federal environmental officials changed the rules and stacked the deck against the company.

“How many times were DEC personnel at the Tonawanda Coke facility?” Gregory F. Linsin, a lawyer for the company, asked a state official who testified Wednesday during the trial in Buffalo federal court.

“At least once a year,” said Larry Sitzman, a regional air pollution control engineer at the state Department of Environmental Conservation.

“Would it surprise you to learn that, during the period from 1997 to 2009, it was 22 times?” Linsin asked.

“No,” Sitzman said.

Sitzman’s testimony is the latest development in the two-week-old trial that is almost certain to fuel debate over the health of residents living in and around Tonawanda’s industrial corridor.

State health officials recently released data showing elevated levels of certain cancers and birth defects among those residents, but the officials have stopped well short of identifying a cause.

An earlier study by the state found concentrations of benzene and formaldehyde to be much higher in the Tonawanda area than other industrial and urban areas.

Sitzman, who was not the state’s primary inspector at Tonawanda Coke, was nevertheless put in the position this week of trying to explain why the bleeder valve escaped regulatory scrutiny for so long.

While testifying, he found himself facing questions about why inspection after inspection found Tonawanda Coke in compliance with state and federal clean air regulations.

“Everything appeared to be OK,” a state inspector wrote in 2003. “All operations were in compliance,” another inspector wrote in 2008.

A few months later, all of that changed when a six-day inspection by state and federal officials resulted in allegations that the valve, a pressure-relief vent, was illegally releasing coke-oven gas with benzene into the air.

Prosecutors say the valve, which is the subject of six of the 19 felony charges against Tonawanda Coke, was emitting toxic gas in greater amounts and at a greater frequency than what the company claimed.

From the start of the trial before Chief U.S. District Judge William M. Skretny, Linsin has argued that if inspectors were not aware of the valve, they should have been.

He reminded Sitzman that long before the 2009 inspection, Tonawanda Coke gave the state a report identifying the valve as part of the plant’s coke-oven gas line.

“They provided information in 2003 that there was a pressure-relief valve on the coke-oven gas system, correct?” Linsin asked.

“Correct,” Sitzman said.

Sitzman was asked whether he offered advice on how to stop the valve’s emissions and whether the company ever followed through on that advice.

He said he did exactly that during the 2009 inspection.

“In the months that followed, you decided the PRV [pressure-relief valve] was no longer releasing gas, isn’t that right?” asked Rodney O. Personius, a lawyer for company executive Mark L. Kamholz, another defendant in the case.

“I believe that to be true,” Sitzman said.

Prosecutors countered by trying to explain why it took so long for state inspectors to raise concerns about the bleeder valve. They noted that the valve is in a location high above the roadway used by inspectors and suggested that Kamholz and others purposely kept the valve from going off during inspections.

“How much advance notice did you give Mark Kamholz?” Assistant U.S. Attorney Aaron J. Mango asked, referring to the state’s inspections.

“A day or two,” said Sitzman.

Mango also wanted to know if Sitzman ever asked Kamholz about benzene levels in the air around the plant and whether Kamholz ever identified the bleeder valve as a possible source.

“During the inspection, did you tell defendant Kamholz about your concerns about benzene?” he asked.

“Yes,” Sitzman answered.

“Were there any sources of benzene brought to your attention by defendant Kamholz?” Mango asked.

“No,” Sitzman said.

In addition to Clean Air Act violations, Tonawanda Coke and Kamholz are accused of violating a federal law known as the Resource Conservation and Recovery Act, which covers the storage and treatment of coal-tar sludge, one of the byproducts of making coke.