By Peter Apps LONDON, April 23 (Reuters) - An investigation by Sri Lanka into the killing of 17 tsunami aid workers was seriously flawed, a group of international lawyers said on Monday, warning the rule of law on the Indian Ocean island was under threat. The 17 local Sri Lankan staff from French aid group Action Contre la Faim were found shot at close range in the northeastern town of Mutur last August after fighting between the government and Liberation Tigers of Tamil Eelam (LTTE). They had been working on rebuilding after the 2004 tsunami. The International Commission of Jurists -- a collection of international lawyers based in Switzerland -- accused the government of a lack of impartiality, transparency and effectiveness in its investigation, saying police had not even bothered to interview a single soldier nearby at the time. "We are very disappointed," ICJ Secretary General Nicholas Howen told Reuters in a telephone interview. "These are grave concerns that are being echoed not only in Sri Lanka but internationally. Clearly this is a great test of the ability of the criminal justice system in Sri Lanka to deliver justice." The ICJ and other human rights groups say abuses, abductions and extrajudicial killings have soared since a 2002 ceasefire effectively collapsed last year and the rebels resume their fight for an independent ethnic Tamil state. Each side blames the other for atrocities, but deny responsibility themselves. "It is clear that there have been severe provocations by the LTTE," Howen said. "What is disturbing is that the government has not adequately responded within the rule of law." TROOPS BLAMED The killing of the aid staff was the worst single attack on relief workers since the 2003 bomb attack on the United Nations compound in Baghdad, but attracted much less attention because no foreign nationals were involved. Most of the dead aid workers were from the Tamil minority, the aid agency had worked frequently in rebel areas and the families immediately pointed the finger at the Army. Denied access to the scene, Nordic ceasefire monitors ruled the murders a violation by the government. The government has always denied involvement in the killing, has defended the integrity of its investigation and has launched a presidential probe into the killing and other high profile reported abuses. "I feel we ought to give the independent commission time to investigate," Foreign Secretary Palitha Kohona said, adding he had not seen the ICJ report yet. The ICJ says the jury was still out on whether the commission would do its job, but said that in any case the criminal justice system itself should probe better, with a truly independent investigation and proper protection for witnesses. Along with other human rights groups, it is also calling for United Nations human rights observers to be sent to the island -- a demand that the government has so far refused. With around 4000 people dead since late 2005 and peace talks seemingly still distant, few diplomats or analysts expect little other than more killing. (Additional reporting by John Ruwitch in Colombo)