Forensic examination into the July 11 Mumbai serial train blasts have revealed use of RDX, traces of ammonium nitrate and which was triggered possibly by a timer device.

The investigations also led to detection of lubrication oil, which could create black smoke and add to the confusion among passengers to trigger off a stampede and choking among them, ATS said in Mumbai on Monday.

The results of the forensic tests took time as residual samples of explosives collected from the blasts spots were miniscule and forensic experts took a long time to reach a conclusion, ATS chief K P Raghuvanshi told reporters.

He said help of the Forensic Science Laboratory, Hyderabad, and the bomb unit of National Security Guards was also taken.

He said forensic experts did not find traces of gelatine in any of the samples tested so far.

Stating that investigations were at a critical stage, Raghuvanshi, without naming any organisation, said probe suggests that the blasts were engineered by elements "beyond borders" and executed with the help of local population.

However, no arrest has been made so far, he said.

Nearly 200 people were killed and over 700 injured after seven blasts rocked commuter trains on the Western line on July 11.

Raghuvanshi also denied media reports that two persons have been arrested from Nepal and brought to Mumbai for interrogation.

He also brushed aside media reports that two persons from central India have been arrested as they have confessed to have participated in the Mumbai train blasts.

"I am in touch with the police units of Gujarat, Madhya Pradesh, Andhra Pradesh and Delhi and there is no information so far on this aspect," he said.

An ATS team has reached Agartala to interrogate 11 residents of Mumbra town in Thane district who were held on the border areas of Tripura, he said.

"If it is found that the suspects have knowledge of any pre-blast or post-blast developments, they will be brought to Mumbai," he said.

Raghuvanshi said the ATS has initiated a probe to find out details about a suspected terrorist site from UK, and has traced origins of the site. "Such attempts have been made earlier too and we are keeping a tab on it," he said.