Four chemists who worked at a New Jersey manufacturer of generic drugs pleaded guilty Thursday to the pharmaceutical industry's version of "cooking the books."

The four men, who were supervisors at Able Laboratories, admitted in federal court in Newark that they falsified and altered data generated in tests of generic drugs required by the Food and Drug Administration. In addition, one of the men, Shashikant Shah, 65, pleaded guilty to one count of securities fraud, having reaped $909,000 in profit from stock sales during the time when the company was falsifying test results.

The Securities and Exchange Commission also filed inside-trading charges against him.

In court, Shah and another defendant, Jyotin Parikh, 56, said they were acting at the direction of the firm's CEO, Dhananjay Wadekar.

Able Laboratories was a manufacturer of generic versions of popular drugs such as Percocet, Aleve, Naprosyn and other medications. Prior to the company's admission in May 2005 that it had discovered irregularities in its quality-control systems, Able employed 500 people and had more than $100 million in annual revenue. It distributed more than 1 billion doses of generic medication to wholesalers around the country annually.

From 2002 to 2005, the company seemed to prosper, and its stock price rose from 55 cents to $26.

But after the testing problems were disclosed on May 18, 2005, the company's share price dropped nearly 75% in one day, from $24 to $6.36, wiping out $340 million in market capitalization. Wadekar resigned. A few months later, the company filed for bankruptcy. It later was liquidated.

"The damage from the fraud at Able Labs was devastatingly complete," said Christopher Christie, the U.S. Attorney for New Jersey, in a statement. "Consumers were put at risk, a company that employed 500 people was destroyed, and shareholders were left with nothing in the end."

Robert Kirsch, the assistant U.S. Attorney prosecuting in the case, said that Parikh and Shah "admitted their participation in a secret project under the direction of the CEO, which involved the falsification and manipulation of chemists' notebooks and binders."

Kirsch would not elaborate on whether the government would bring charges against Wadekar or other senior executives who sold stock during the run-up in Able's share price. "The investigation is active and ongoing," he said.

In addition to Parikh and Shah, Ashish Macwan, 47, and Jose Concepcion, 57, each pleaded guilty to one count of conspiracy to distribute misbranded and adulterated drugs. Henry Klingeman, an attorney for Macwan, said his client "recognized the gravity of his conduct from the moment when he was contacted by the government, and he immediately began cooperating." He said he hopes Macwan's cooperation with investigators would be recognized at sentencing.

Attorneys for the other defendants did not return phone calls, and Wadekar, the former CEO, could not be located.