He is the author of the book De kalder mig terrorist (They call me Terrorist). He claims to have been involved in a 14-year journey from 1982 that involved building schools in Guatemala, agricultural projects in India and attending tiger conferences in Russia.
Around 1995 he was involved in humanitarian work for the people in West Bengal, which at that time was governed by the Communist Party of India. The authorities claim that he was involved in smuggling of weapons. He admits as much himself in his book They Call me A Terrorist, claiming that he made the weapon drop from a plane purchased in Latvia. Apparently his motives were to help the local communities in West Bengal fight the oppression brought on them by their government. A total of 4 tonnes of arms were trafficked during his dealings in West Bengal. Additionally, in 2010 Danish police confiscated two forged and expired passports with British names and the photo of Niels whilst searching his home.
As of 2009 he is considered the second Dane together with Camilla Broe at risk of being extradited to a country outside the European Union where he faces a possible death sentence. Camilla Broe was indicted for the importation of narcotics into the United States and while extradited and delivered to US custody was later returned to Denmark when the statute of limitations barred an expeditious trial.
On 9 April 2010, Danish police arrested him in order to start an extradition trial. Based on experiences from the fiasco in the Camilla Broe case the prosecution will demand detention but will accept a confiscation of his passport(s) if the court feels that the rather lengthy extradition process will prevent him from keeping contact with his family. The extradition agreement between Denmark and India has been changed so he will have to be returned home no later than 3 weeks after a sentence has been given at the expected trial abroad.
On 1 November 2010, The Hillerød Court overturned the Justice Ministry decision to extradite Niels Holck based on the expected inability to protect him against "rough treatment". On 14 November 2010, the Indian government told that they would try to get him extradited despite the clear ruling of the court.
Terms of the extradition agreement
- No execution of a possible death sentence (Unchanged from the previous agreement).
- Favouritised and human treatment of the extradited person while being detained abroad (changed due to the food condition experienced by Camilla Broe in the United States).
- Once convicted the person has to be returned to Denmark in order to serve the sentence no later than 3 weeks (Previously 6 months) after the conviction.
Amnesty International did however criticize the extradition based on lack of trust that countries like India doesn't have the resources to fulfill their part of such agreements. The United Nations Special Torture Rapporteur was quoted for the statement that the Indian guarantees that the man will not be tortured are worthless.
In April 2010, the news agency Press Trust of India reported that the Danish government has been beaten to submit Niels Holck not only to the trial but also to serve out his entire sentence in India.
In September 2010, it was reported that the court system in India is not committed to follow the terms of any extradition agreement due to the ruling in the case against another extradited underworld don Abu Salem who was extradited from Portugal to India. This ruling from the Supreme Court of India can influence the extradition case against Holck because the agreement between Denmark and India mandates that Holck cannot be convicted to death 
On 30 June 2010, the High Court of Eastern Denmark ruled that he should not be extradited to India based on the risk of torture by the Indian authorities.
Diplomatic rift between Denmark and India
On 16 August 2011, India decided to cut their connection to Denmark. Also, the massive Danish aid came under intense monitoring by the authorities in India. This was a consequence of the fact that Denmark would not extradite Holck and the wish of the Danish government to secure a more fair, civilized and direct distribution of the aid.
- Camilla Broe - the first Danish citizen to be extradited to a country outside the European Union viz. United States for drug trafficking.
- Clotilde Reiss - a French student who was arrested (and released after about 10 months) by Iran for being an agent of the French Secret Service
- Abu Salem - an Underworld don who was extradited to India from Portugal
- Gary McKinnon
- Udvikling, book review by Signe H. Damkjaer, the foreign department of Denmark
- Up in arms: The bizarre case of the British gun-runner, the Indian rebels and the missing Dane, By Peter Popham, The Independent, March 6, 2011
- "Niels Holck (f. 1961)" (in Danish). bibliotek.dk. Retrieved 30 January 2015.
- Holck, Niels; Kyroe, Oejvind (2008). De kalder mig terrorist [They call me Terrorist] (in Danish). Copenhagen: People's Press. 270 pages. ISBN 978-87-7055-275-2. OCLC 474109214.
- "Græsrod med hang til husbåde" (in Danish). fyens.dk. Retrieved 17 April 2005.
- Vester, Hilmar (April 10, 2010). "Våbensmugler anholdt med falske pas" [Weapons smuggler arrested with forged passports] (in Danish). Danmarks Radio. Retrieved 30 January 2015.
- Camilla Broe Acquitted but Stuck in the US Archived 2010-03-02 at the Wayback Machine.
- Skal straffes i Indien, Jan Lauridtsen, BT, April 9, 2010 (in Danish)
- Dommere har det sidste ord om udlevering, BT, April 9, 2010 (in Danish)
- Court stops Holck extradition to India, by Julian Isherwood, Politiken, November 1, 2010
- Indien opgiver ikke Niels Holck, by Jeppe Lykke Hansen, TV2, 14 November 2010
- Dansker udleveres til domstol i Indien, Danmarks Radio, April 9, 2010 (in Danish)
- Hård kritik af udvisninger, by Ole Damkjær, Berlingske Tidende, 12 April 2010 (in Danish)
- Dangerous deals: Europe's reliance on "Diplomatic Assurances" against torture, Amnesty International, April 2010
- U.N.: Indian torture promise useless, by Julian Isherwood, Politiken, April 19, 2010
- Kim Davy could be extradited to India by July-end, Press Trust of India
- Abu Salem can face death penalty: SC, Indian Express, 10 September 2010
- Indisk løftebrud kan lukke Holck-sagen, by Carsten Norton, Ekstra Bladet, 22 September 2010
- No extradition due to India torture fears, by Julian Isherwood, Politiken, June 30, 2011
- India closely monitoring Danish aid, by Julian Isherwood, Politiken, 21 November 2011