The four Chinese citizens pleaded guilty to charges including conspiracy to harbor aliens, money laundering and tax evasion in connection with the $700,000 enterprise, which sold Taiwanese, mainland Chinese and Korean women for sex out of residential brothels in San Jose, Santa Clara, Sunnyvale and San Mateo. A fifth defendant pleaded guilty Feb. 1.

Those who entered guilty pleas in U.S. District Court in San Francisco are: Xiao Feng Shen, 35; Jia Jing Chu, 34; Yan Song, 42; and Ming Sun, 32, all of San Jose; and Ai-Ching Chang, 45, of Sunnyvale. Sentencing is scheduled for May 3. Conspiracy to harbor aliens carries a maximum 10 year prison term and $250,000 fine. Money laundering brings a maximum 20-year prison term and $500,000 fine and tax evasion convictions can result in a maximum 5 year prison term and $100,000 fine.

The investigation began in fall 2004, when one of the defendants tried to bribe a San Mateo police officer to dismiss pimping and pandering charges against a co-defendant.

Federal investigators from Immigration and Customs Enforcement and the Internal Revenue Service discovered the ring leaders were shuffling dozens of women every 10 days between 10 residential brothels. Customers found the unlicensed massage parlors through the Internet and word of mouth.

Brothel operators controlled the women's access to the outside world by keeping them in rooms inside the brothels or in nearby apartments they owned. They kept the curtains closed, provided sex supplies and moved the women around in their own cars or by drivers who worked for the illegal network. They also kept a portion of each woman's earnings.

After federal raids last October, officials detained 31 women and interviewed them to determine if they were sex trafficking victims forced to work against their will. Nineteen were deported, but 12 remained to testify against the brothel owners. Now those 12 will undergo immigration proceedings to determine their residency status.

"We want to make it perfectly clear that the degradation of women will not be tolerated in our communities, and we will be proactive in our efforts to arrest those who prey upon the plight of other human beings," San Jose Police Chief Rob Davis said in a written statement.