There was nothing unusual about the faded yellow house on Belair Road. Its inhabitants mostly kept to themselves and were rarely seen outdoors. Only after federal and state investigators raided the site in May did neighbors learn that the building, which billed itself as an Asian massage parlor, actually served as a front for what authorities say was a sex-trafficking ring that forced women to have sex with as many as 15 to 20 men a day.

All the women were Asian immigrants who, prosecutors say, were shuttled from house to house in what is alleged to have been a multi-state prostitution network along the Eastern seaboard; some of the women apparently were unaware even what state or city they were in. Authorities arrested five people in connection with an investigation into suspected brothels in five states and charged them with conspiracy, racketeering, coercion, money laundering and other crimes.

The raid on the house in Overlea, just across the city line in Baltimore County, threw a rare spotlight on the largely hidden world of human trafficking in Maryland. Neighbors say the Elite Spa operated under their noses for years without anyone realizing what it was or that it existed. Even when they noticed something suspicious, such as the steady stream of men visiting the house, they didn't recognize a crime being committed or know who to call.

And the Elite Spa may represent only the tip of the iceberg. Asian massage parlors are common across the Baltimore-Washington region, and authorities say they often have no way of knowing what goes on inside them without conducting lengthy investigations. A group that advocates for women victimized by human traffickers estimates there may be more than 4,000 such establishments nationwide.

Last year, authorities shut down half a dozen suspected brothels in Baltimore County. Yet the criminals know that laws against trafficking are difficult to enforce, while the potential profits are enormous. Wiretapped conversations between the alleged traffickers in the Overlea investigation suggest they were taking in $1 million a year. Beyond Maryland, human trafficking is a $9 billion-a-year global industry that ranks among organized crime's most profitable enterprises.

Maryland has made important strides in recent years in combating human trafficking. In 2007, the General Assembly passed a law changing child sex trafficking from a misdemeanor to a felony punishable by up to 25 years in prison, and this year it approved legislation allowing trafficked victims to have a conviction for prostitution expunged if they can prove they were lured into the trade by force, fraud or coercion.

But tougher laws are still needed against criminals who traffic in adult women, which presently is only a misdemeanor. Authorities should also be able to go after those who knowingly aid and abet human traffickers, such as brothel owners, doorkeepers, drivers and bartenders. Without their tacit cooperation, it would be impossible to keep the violent world of forced prostitution and sexual slavery hidden from view, along with the viciously exploited women and girls who are its main victims.