A US Congress team has been meeting relatives of some of the hundreds of Mexican women whose unsolved murders have caused a national scandal.
Many bodies have been discarded in desert areas
The six members of the Congress delegation are in the border city of Ciudad Juarez, the region where most of the bodies of the women have been found over the last 10 years.
The Mexican authorities have failed to find the perpetrators and have been criticised for apparent bungling.
The delegation say they will be making a report on their visit on Monday.
Democrat Hilda Solis, from southern California, said: "We are here to support the families of the victims, gather data, make an investigation.
Solis and her colleagues met for more than three hours with a group of 20 relatives of the slain women.
Since 1993 the bodies of the dead girls have been found buried in dusty desert graves or left on roadsides.
It is not known whether the murders - many involving sex crimes - have been carried out by a serial killer or killers, or if criminal gangs are responsible.
Figures available in Mexico put the number of dead at more than 250. But according to human rights activists more than 370 women have been murdered in Ciudad Juarez and more recently the state capital, Chihuahua.
The victims have been mostly poor maquiladores - seamstresses working the numerous assembly lines - and many of the corpses were found mutilated and strangled, says the BBC's Claire Marshall in Mexico City.
Representatives of civil groups such as "Bring Our Daughters Home" told the visiting Congressmen they had had difficulty getting the authorities to investigate the cases.
Eric Olsen, an Amnesty International representatives who accompanied the lawmakers to Mexico, said the aim of the trip was to help ensure that justice is done.
The London-based group Amnesty International has accused Mexican police of negligence and fabricating evidence in their investigation of hundreds of sex killings of young women.
Judicial officials have blamed drug traffickers, domestic violence and serial killers but police and prosecutors are in a "culture of denial", says Amnesty.
The Congress delegation will try to pressure Vicente Fox's government to focus more attention on solving the murders.
Rupert Knox, a researcher for Amnesty International who has written a book on the subject, said the US also has economic interests in Chihuahua.
He said part of the problem has been the autonomy attached to Chihuahua, which detaches it from Mexico.
But he added that the Mexican authorities tardiness in solving the crimes amounted to "ignoring a pattern of violence against women.
"The prosecution service has failed greatly," he added.