ARAU: Police has discovered 139 graves which are believed to contain human remains at a 3km square area in Wang Kelian here. Inspector General of Police (IGP) Tan Sri Khalid Abu Bakar said a highly decomposed body was also found at one of the 28 camps. "It's a very sad scene... To us even one is serious and we have found 139. We do not know how many bodies are in each grave," Khalid told reporters at a news conference here. "We are working closely with our counterparts in Thailand. We will find the people who did this." The grisly find follows the discovery of similar shallow graves on the Thai side of the border earlier this month, which helped trigger a regional crisis. After a crackdown on the camps by Thai authorities, traffickers abandoned thousands of migrants in rickety boats in the Bay of Bengal and Andaman Sea. "We are shocked by the cruelty," said Khalid, describing conditions at the 28 abandoned camps where the graves were found in an operation that began on May 11. Pictures of the camps shown to journalists by Malaysian police showed basic wooden huts built in forest clearings. Khalid said ammunition was found in the vicinity and added there were signs that torture had been used, without elaborating. Metal chains were found near some graves. He said the camps were just 500 metres away from the Malaysian-Thai border and some 100 metres away from where Thai authorities discovered mass graves on May 1. He also said the camps were discovered after gleaning information from the arrest of 37 individuals this year. Police initiated a 49.5 km square "sweep and comb" operation starting May 11 from Tangga Seratus to Wang Kelian, leading to the discovery. Khalid said 31 forensic personnel will begin exhuming the graves to obtain further information. "We will open sudden death reports (SDR) first, then exhume the remains and determine the cause of death," he said in a press conference today and stressed action would be taken against those responsible. Khalid said the camps came in various sizes with the largest able to hold 300 individuals. He said the camps were meant to detain people as wooden fences were built around them. "We also found a sentry post," he said Khalid said the search of the area show no paths for smuggling activities, whether human or goods. He suggested the persons being smuggled could have entered via Bukit Kayu Hitam or Wang Kelian. "They also could have entered through the ' laluan tikus' (small gaps) in the border fence," he said adding that measures to enhance security have seen been initiated. Khalid also rebutted questions on border security, saying the areas where the camps were found are in a hilly area. He said the sweep operation area was divided into three sectors - A,B,C - with the first two being very hilly, while the last sector was a village. He said there was also no indication nor information on human smuggling activities in the area. "The places where we conduct patrols are in areas near human settlements," he added. On efforts to curb human smuggling, Khalid said 20 individuals have been charged under the Anti-Trafficking in Persons and Anti-Smuggling of Migrants Act (2007) this year and 60 in 2014. He said action has been taken against 10 persons under the Prevention of Crime Act ( POCA ) (2013) this year and two in 2014. Khalid added police this year detained 1,305 illegal immigrants in Perlis, 853 in Kedah, 498 in Perak and 88 in Kelantan. He said in 2014, 1,914 illegals were detained in Perlis, 2,014 in Kedah, 962 in Perak and 3,462 in Kelantan. Meanwhile, Prime Minister Datuk Seri Najib Abdul Razak said he was deeply concerned about the graves found on Malaysian soil purportedly connected to people smuggling activities, Bernama reported. "We will find those responsible," he vowed via his Twitter account.