In November, Merck settled charges of drug marketing and safety fraud for $950 million. A month earlier, Oracle agreed to pay $199.5 million after being accused of overbilling the government for software.

The difficulties of prosecuting executives were highlighted last week in New York, where a federal jury acquitted a Citigroup manager who had been involved in selling an exotic financial security involving residential mortgages. The manager, Brian H. Stoker, was charged with falsely describing Citigroup’s role in selecting the assets in the portfolio and failing to disclose that Citigroup was betting against the investment.

The jury cleared Mr. Stoker in part because the bank had given investors fine-print materials that apparently warned them of the investment’s risks. In a rare move, though, the jury sent a note to the Securities and Exchange Commission after reaching its decision, urging the agency not to give up. “This verdict should not deter the S.E.C. from investigating the financial industry, to review current regulations and modify existing regulations as necessary,” the jury wrote.

Lawyers say the government is more likely to go after companies because of their deep pockets. Civil cases against businesses can often produce substantial financial awards without the risk inherent in a trial. Civil charges also have a lower burden of proof than criminal charges and can reap triple damages. By one estimate, the government recoups $15 for every $1 spent on a civil case against a company.

But a top government enforcement official gave another reason, saying it was often too difficult and expensive to find evidence that clearly linked individual actions to corporate wrongdoing. Senior executives in particular are often insulated from day-to-day decisions, the official said, and have learned to steer clear of e-mails or other evidence that might prove that they knew the company was breaking the law. The official spoke on the condition of anonymity because more companies and executives were expected to be taken to court.

The Justice Department said its prosecutors assessed how strong the evidence was and the likelihood of a successful trial in deciding whether to charge individuals. Even if a company has settled a case, it said, an investigation of individual conduct can continue and might eventually result in charges.