|Investigators found a note between two bodies discovered on Wednesday saying more remains will be found [EPA]
Forensic investigators have begun searching for more hidden graves on Mexico's west coast after they found a sign next to bodies uncovered earlier that indicated that they would find the remains of 20 men.
Authorities believe the 18 badly decomposed remains found on Wednesday belong to a group of Mexican tourists who were abducted a month ago near Acapulco, a popular Pacific coast resort in Guerrero state.
"We have not said that it is them, but we have not discounted that either," David Sotelo, the attorney general for Guerrero, said on Thursday.
Investigators used long metal poles to probe the ground in a coconut palm grove in the community of Tuncingo, just east of Acapulco.
An anonymous telephone tip led police to the bodies of two men on Wednesday.
The 20 men, most of them mechanics, disappeared on September 30 after setting out from the neighbouring state of Michoacan for a holiday in Acapulco.
Before the first two bodies were found, a video posted on the video-sharing site Youtube showed two men with their hands apparently tied behind their backs telling an unseen interrogator that they killed "the Michoacanos" and buried them in the area.
"The people they killed are buried here," read the sign left between the two bodies, signed by Acapulco's Independent Cartel.
The group is a little-known drug gang that has been claiming responsibility for killings in the area over the past two months.
It is believed to be a breakaway faction of the Beltran Leyva gang, whose leaders have recently been killed or captured.
The two men interrogated in the video appear to be members of a rival faction. They were recorded as saying that the killing was an act of revenge against La Familia, a powerful drug gang based in Michoacan.
Authorities have, however, not determined a specific motive behind the mass killing.
In his comments on Thursday, Sotelo, the Guerrero attorney general, said he had asked officials in Michoacan "to put us in contact with the relatives of the people who were kidnapped so that they can identify whether the bodies are those of their relatives".
His remarks came on a day of more violence in Acapulco. Four city police officers were shot dead in two separate attacks, authorities said.
The officers were attacked while on patrol by armed men travelling in 4WD vehicles, Guerrero state's public-safety department said in a statement.
A fifth officer was wounded after being shot while directing traffic in the city.
Mass killings, often connected to the drugs trade, have become more frequent in Mexico.
More than 29,000 people have been killed since the Mexican government launched a military crackdown on criminal gangs in December 2006.