LONDON (Reuters) - David Carruthers, chief executive of online gaming group BETonSPORTS, was detained by U.S. authorities while changing planes in Dallas, the company said on Monday, sending its shares down more than 15 percent.

Carruthers, who has visited the United States in the past and was mounting a campaign to regulate -- and legalize -- Internet gambling in the country, was in custody in Fort Worth, Texas, a company spokesman said. BETonSPORTS is reviewing the situation, spokesman Kevin Smith said.

A federal grand jury in the Eastern District of Missouri returned a 22-count indictment charging 11 individuals and four corporations, including BETonSPORTS and three companies based in Florida, on various charges of racketeering, conspiracy and fraud, the U.S. Department of Justice said in a statement.

The founder of BETonSPORTS.com, Gary Stephen Kaplan, 47, was also charged with 20 felony violations of federal laws, it said. A warrant has also been issued for the arrest of Kaplan, who resides in Costa Rica.

The United States will seek extradition of the defendants to St. Louis for prosecution, the Department of Justice said.

"While changing flights in the United States en route from the UK to Costa Rica, David Carruthers, chief executive of BETonSPORTS Plc was detained by U.S. federal authorities," the company said in a statement.

Earlier a spokeswoman for BETonSPORTS, which runs a large part of its operation from the Costa Rican capital, said the incident happened on Sunday night when Carruthers was on his way to San Jose.

The United States has also filed a civil complaint to obtain an order requiring BETonSPORTS to stop taking sports bets from the country and to return money held in wagering accounts to account holders in the United States, the Justice Department said.

U.S. District Judge Catherine Perry issued a temporary restraining order on Monday, with a hearing in the civil case requested within 10 days.

"Illegal commercial gambling across state and international borders is a crime," U.S. Attorney Catherine Hanaway of the Eastern District of Missouri said. "This indictment is but one step in a series of actions designed to punish and seize the profits of individuals who disregard federal and state laws."

Last week Carruthers told Reuters he thought that a U.S. Republican-written, House of Representatives-approved bill to crack down on Internet gambling by banning banks and credit card companies from processing the payments was bound to fail due to the mountainous backlog of other U.S. legislation.