Kiessling, who spent around 13 years in Pakistan during which he studied structure and work of ISI, claimed the Pakistani intelligence agency is one of the best in Asia and had a staff strength of around 3,500 in 2004 when RAW's strength was around 7,000.
He was speaking at the Observer Research Foundation after release of his book "Faith, Unity, Discipline: The ISI of Pakistan".
He said the book is an updated version of his previous volume on work of ISI and India's Research and Analysis Wing (RAW) published in German in 2011.
"The book spans the period from 1948, the period of establishing of the agency (ISI) to 2015. This work is based on my observations and experiences and I have used historical method. There were several factors which led me into doing this book," he said.
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"While researching the book, I met several top officials who had been associated with ISI, from DG-rank to others, and one thing I found out was that the archiving of information, about it, was not done properly... Also, in 2004, ISI had 3,500 staff," he said.
"I am not in favour of conspiracy theories. There have been many conspiracy theories around the September 11 attack in the US. So, this was another one... Lot of people also still think that US astronauts never landed on the moon and it was staged in a desert. So, let's not get into conspiracy theories," he said.
The author also said that on the Kashmir issue, "From the beginning I was very careful on this subject."
The author says his contacts for researching the book were senior retired army officers.
On another question on the strength and weakness of ISI, he said, "It is one of the best intelligence agencies, 'second to none' they call themselves too. But, their weakness, if you ask me is that they do not take into account what is happening around the world."
"I personally feel that like the DGMOs, the chiefs of RAW and ISI should meet to work out a sensible solution," he said.