A five-day operation aimed at rounding up illegal immigrants with criminal histories has netted 274 arrests in the Dallas-Fort Worth area – including 99 convicts.

"It's very significant. It may be the largest of such operations locally," said Carl Rusnok, spokesman for U.S. Immigration and Customs Enforcement in Dallas.

Those detained during the operation, which ended Friday, include 233 men and 28 women. Some of those arrested have been accused of sexual assault of a child, dealing drugs, theft or drunken driving.

About half have been returned to Mexico. Authorities also detained 13 juveniles, some of whom might be turned over to the federal Office of Refugee Resettlement.

Most of those who haven't been returned to Mexico are in deportation proceedings; their home countries include Colombia, Cuba, the Dominican Republic, El Salvador, Guatemala, Honduras, Mexico, Nigeria, Romania and South Korea.

Among those arrested is 40-year-old Gustavo Noe Castro of Mexico. In 2000, Mr. Castro was arrested and convicted in Dallas of assaulting a family member. That same year, Mr. Castro was convicted of possessing a controlled substance.

He has been arrested three times on suspicion of drunken driving, according to ICE.

Another man, 40-year-old Francisco Rafael Trejo of El Salvador, was convicted of unlawfully carrying a weapon in Dallas in the early 1990s. He was also convicted of possessing a controlled substance in 2003 and has been arrested four times on suspicion of drunken driving in Dallas.

Mr. Rusnok said one possible reason that the men were not detained and sent back to their native countries sooner is that ICE might not have been notified of their convictions.

Mr. Rusnok said the operation targeted convicted criminals who have been ordered to be removed by a judge. In the process, authorities came across others who were in the country illegally.

Assisting in this week's arrests were the Dallas County constable and the Dallas, Irving, Fort Worth, Farmers Branch, Carrollton, Arlington and Blue Mound police departments.

In April, federal agents in Dallas arrested 41 illegal immigrants accused of crimes, including a child predator. An additional 107 illegal immigrants, including seven juveniles, were apprehended.

Last year, ICE deported 197,000 people to their home countries, Mr. Rusnok said. Of those, 88,600 were convicted of crimes.