LOS ANGELES — Two gangs that originated on the streets here have grown so large in El Salvador that there are two prisons in that country devoted exclusively to their members, one for each gang, according to officials who traveled there recently to meet with the local authorities.

That is just one measure of the way gangs in this city with the worst gang problem in the United States have bolstered their presence in Mexico and Central America, where they attract new members eager to come here. The growth in their transnational networks has made these criminal organizations all the more worrisome, officials say.

“These gangs are the new and emerging organized crime in America,” said Bruce Riordan, director of anti-gang operations for the Los Angeles city attorney’s office.

Last week the federal government and Los Angeles County undertook a joint attack on transnational gangs by charging 23 incarcerated gang members with the felony offense of re-entering the United States after being deported. The men, in their 20s and 30s, had been awaiting release from state prisons or city jails where they were serving time for a variety of offenses. They now face up to 20 more years in federal prison if convicted.

Prison sweeps like last week’s are the latest phase in a two-year-old program to identify transnational gang members in Southern California who have violated immigration law, said Jim Hayes, a field officer here for Immigration and Customs Enforcement.

Though a majority of gang members are American citizens, the large number of deportations of those who are not has facilitated cross-border movement that abets transnational gang expansion, said Gary Hearnsberger, chief of the Hardcore Gang Division at the Los Angeles County district attorney’s office.

Part of the reason is that after deportation without federal prosecution, gang members are generally not subject to penalties in the countries to which they are expelled, officials say. “They get a chance to hang out in another country for a while, then come right back,” often having recruited new members, Mr. Hearnsberger said.

With regular prison sweeps, however, those who have violated deportation will be less likely to slip through the cracks. After a felony conviction for the offense, they will not only serve a federal sentence here but then be released into the custody of law enforcement officials in their home countries.

Some of the men charged last week had been deported as many as seven times. Deportees carry with them the gang tattoos and the uniform of white T-shirts, thin belts and baggy pants with split cuffs. Those marks of gang affiliation are alluring to potential recruits in Latin American countries seeking work and support networks.

The deportees “don’t typically just quit their gang attitudes and gang associations,” Mr. Hearnsberger said. “They take those with them,” and as a result “you might be transporting their gang business to another country.”

Mr. Riordan traces the growth of transnational gangs to the Sept. 11 attacks and a resulting shift in federal law enforcement resources.

“In the late 1990s, we were having a lot of success convicting the leadership of the Mexican Mafia, 18th Street Gang and the Rolling 60s,” he said, referring to three of the largest of the estimated 1,000 gangs in Los Angeles County. “The events of 9/11 led to a shift of resources away from domestic violent crimes to terrorism.”

That shift, along with a vacuum created by a decline in traditional organized crime networks, allowed transnational gangs to gain a foothold in the narcotics trade and human trafficking, Mr. Riordan said.

Copyright 2007 The New York Times Company. Reprinted from The New York Times, National, of Wednesday, December 26, 2007.