Question: The murders commited by Colin Howell were the subject of an ITV drama series based on murders commited in what year?

Context paragraphs:
Neil Dudgeon: Neil Dudgeon (born 2 January 1961) is an English actor who, from 2011, has played DCI John Barnaby in the ITV drama series "Midsomer Murders".  He replaced the retired John Nettles in the lead role.

Andrew Buchan: Andrew Buchan (born 19 February 1979) is an English stage and television actor known for his roles as Mark Latimer in the ITV drama "Broadchurch" (2013–17), as Scott Foster in the BBC political drama "Party Animals" (2007), as John Mercer in ITV drama series "The Fixer" (2008–09), and as William Garrow in BBC period drama "Garrow's Law" (2009–11).

Crime Stories (UK TV series): Crime Stories is a British television crime drama series that aired on ITV from 12 November until 7 December 2012.  Described as "groundbreaking" television, the series was one of a number of series developed by Paul Marquess which makes use of a documentary-style feel and unscripted dialogue.  The series was panned by critics and viewers alike and was axed by ITV after just one series.  The series made use of the set previously occupied by fellow ITV drama "The Bill", for which Marquess served as executive producer from 2002–05.

Death in Disguise: Death in Disguise is a crime novel by Caroline Graham, the third in her popular Chief Inspector Barnaby series, which has been adapted into the successful ITV drama "Midsomer Murders".

Colin Howell: Colin Howell (born 14 March 1959) is a Northern Irish convicted double murderer.  The murders and surrounding story were the subject of an ITV drama series "The Secret" broadcast in April and May 2016.

A Place of Safety: A Place of Safety is a crime novel by Caroline Graham, the sixth in her popular Chief Inspector Barnaby series, which has been adapted into the successful ITV drama "Midsomer Murders".

Death of a Hollow Man: Death of a Hollow Man is a work of detective fiction by Caroline Graham, the second in her Chief Inspector Barnaby series, which has been adapted into the successful ITV drama "Midsomer Murders".

Richard Walsh (actor): Richard Francis Evelyn Walsh (born 8 December 1952 in Lewisham, London) is an English actor, best known for playing fireman Bert "Sicknote" Quigley in the long-running ITV drama series "London's Burning" from 1986 to 2000.  He has also appeared in other well-known British television shows, including "Midsomer Murders", "Doctors" and "Heartbeat".  He is married to fellow actress Sarah Keller with one son.

The Secret (TV series): The Secret is a 2016 Northern Irish ITV mini-series based on "Let This Be Our Secret", Belfast journalist Deric Henderson's account of the double murder of Lesley Howell and Constable Trevor Buchanan, whose bodies were discovered in a fume-filled car in the garage of a property in Castlerock, County Londonderry, in Northern Ireland in May 1991.

Stephen North: Stephen "Steve" North (born 1965) is an English actor.  He had his first major role playing Firefighter Colin Parrish in the ITV drama London's Burning between 1990 and 1993.  He has since appeared in numerous shows on British television, including "Doctor Who Christmas Special 2010", "EastEnders", "The Bill", "The Day Britain Stopped", "Murphy's Law", "Holby City", "Midsomer Murders", "Doctors", "Is Harry on the Boat? ", "" and "Casualty".  North also played one of the two lead roles in the award winning stage play "Meeting Joe Strummer" along with "Emmerdale" actor Nick Miles.  He was the solo performer in the original stage adaptation of Nick Hornby's "Fever Pitch", which ran at The Arts Theatre London in 1995.  He played the role of Ted Narracott in War Horse (play) at the New London Theatre from May 2013 to March 2014 and also in the National Theatre Live broadcast on 27 February 2014 and subsequent Encore screenings.

Supporting paragraphs: Colin Howell The Secret (TV series)
Answer: 1991