ROANOKE, VA. -- Federal grand juries are investigating allegations of price fixing by soft-drink bottlers here and in 10 other locations across the United States, but the motivation of the probe remains a mystery.

Officials with the Justice Department said they are not allowed to comment on why they began looking into conspiracy charges as far back as two years ago in some places. But they have drawn criticism for failing to indict upper-level corporate officials.

Some analysts of the soft-drink industry, meanwhile, say fixing prices would seem to be an almost irresistible temptation to bottlers who dominate regional markets. Others say soda prices have stayed so low in recent years that price-fixing on a grand scale makes no sense.

On June 22, prosecutors got Raymond H. Rissmiller, a former official with Coca-Cola Bottling Co. of Roanoke, to plead guilty to conspiring to fix the prices of soft drinks.

Prosecutors agreed not to charge Rissmiller further in past price-fixing cases. In return, Rissmiller agreed to testify, both before a grand jury and at trials.

He spent an afternoon two days later testifying before a special federal grand jury, according to his lawyer, Charles Dorsey of Roanoke.

Dorsey said Rissmiller's testimony outlined price fixing that had gone on from 1982, when the company had been called Wometco Coca-Cola Bottling Co. of Roanoke Inc., to November 1985. Rissmiller named the names of bottling company officials who allegedly ordered him to communicate with competitors to fix prices, Dorsey said.

Dorsey declined to identify the officials. He said Rissmiller named "two directly and one more indirectly."

No indictments were returned, though federal officials have said evidence will be presented off and on to the grand juries in Roanoke and elsewhere for some time to come.

Officials have said the probe also is taking place in the District of Columbia; Norfolk, Va.; Philadelphia; Sacramento, Calif.; Indianapolis; Cleveland; Atlanta, Macon and Savannah, Ga., and Fort Lauderdale, Fla.

"Most of the cases have involved wholesale pricing as opposed to price fixing at the consumer level, where you and I buy," Justice Department antitrust attorney Robert E. Bloch said. "I expect the investigations to continue for some time."

He and other federal officials declined to say what prompted the scrutiny of the soft-drink bottling business. The overall effort had led to five cases, including Rissmiller's, by late June, Bloch said.

Last fall, General Cinema Beverages of Washington, D.C. Inc., which bottles Pepsi, pleaded guilty to price fixing and was fined $1 million. Later, Armand S. Gravely, the former Richmond, Va., division manager of Alleghany Pepsi-Cola Bottling Co., was convicted of price fixing and obstruction of justice by a federal jury in Norfolk. He was sentenced to four months in jail and fined $15,000.

On June 19, a former president of Alleghany, James P. Sheridan, pleaded guilty in a regional price-fixing scheme and agreed to help prosecutors.

The fifth case is pending. Edward Leon Wynns, a senior vice president for sales for Mid-Atlantic Coca-Cola Bottling Co. of Silver Springs, Md., faces a jury trial on Aug. 26 in Norfolk on counts of perjury and obstruction of justice.

In Gravely's trial, U.S. District Judge Robert G. Doumar repeatedly criticized prosecutors for indicting only Gravely, a middle-level manager. Testimony indicated that he had been following orders in fixing prices.

Dorsey also has questioned prosecutors' motives, saying that his client was a "bit player." In the General Cinema case, he said, bottling executives were given immunity.

"General Cinema, they let the higher ups walk," Dorsey said, adding concerns that his client could become a scapegoat while bosses who allegedly planned the price-fixing could go free.

A spokesman for the Justice Department, Mark Sheehan, said General Cinema executives were told they were "free to cooperate with the investigation and to seek immunity for themselves in doing so."

He declined to say whether any immunity had been granted.

"Whether or not any of them did come in and seek immunity would be a grand jury matter, and I would not be able to tell you," he said.

The Roanoke Coca-Cola bottler's parent company, Coca-Cola Bottling Co. Consolidated of Charlotte, N.C., denied knowing of any price fixing and said there are strict company policies against discussing prices with competitors.