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SIO Corner: Main Lines Of Enquiry (MLOE)

This week we focus on the importance of setting out main lines of enquiry (MLOE) in an investigation

In this series we look at various aspects of life as an SIO. This includes the necessary skill sets for the successful SIO, the management of serious crime investigation and specific elements of investigative practice from initial response through crime scene examination and investigative strategies to dealing with suspects and the media. The articles are excerpts from the 2nd edition of Blackstone's, the 'Senior Investigating Officers' Handbook', written by two highly experienced SIOs (see 'About the Authors' at the end of the article).

Main Lines Of Enquiry (MLOE)

These are a means of setting and focusing clear investigative priorities that aim to establish the facts, find the evidence, and arrest and convict the person(s) responsible for the offence. The SIO formulates a list called the ‘Main lines of enquiry’ in the policy file and ensures that all staff engaged on the enquiry are fully aware of what these are and in which direction the investigation is heading. The enquiry must always remain focused on these and not become too bogged down with other actions and enquiries that are the result of an administration and management system that may sometimes produce workloads that do not reflect the SIO’s main priorities.

Some of these enquiries will be obvious and stand out from the information available and the circumstances of the offence under investigation. For instance, known key witnesses who need to be interviewed quickly, or potential exhibits that can be forensically fast-tracked for the offender’s fingerprints or DNA, or the securing and viewing of some CCTV footage that may provide images of the persons responsible. These will provide the SIO with immediate and clear lines of enquiry. However, in some cases these priorities will not be so obvious and more thought and consideration may be needed as to how the necessary information and evidence is going to be acquired.

In formulating MLOE an SIO has to ask the question: where is the necessary evidence to solve this crime likely to come from?

The answer normally lies in a relatively small list of sources from which such material can be obtained:

A point to note is that for the purposes of formulating main lines of enquiry the SIO is free to use all available information and material (eg intelligence) and not just material that is admissible as evidence.

Any lines of enquiry that appear to have the best potential to reveal useful material and fill one or more of the aforementioned requirements should therefore be prioritized in the MLOE list. Any information relied upon must, of course, be properly evaluated and analysed to determine reliability and suitability. This rule applies to all sources of information, whether from witnesses, electronic/scientific sources, or whatever.

In so doing the A B C principle must be applied:

Assume nothing
Believe nothing
Challenge / check everything

It is always useful to consider the use of an analyst as a professional adviser to help the SIO in evaluating the material available and making an assessment as to accuracy and significance of the investigative information. The analyst can also be tasked with highlighting information or knowledge gaps. These can be identified by applying the 5 × WH + H formula (Who, What, Where, When, Why, and How), explained more fully in Chapter 2.

It must be remembered that setting too many main lines of enquiry may lead to low productivity due to overload. A key skill is to determine and judge the relevance and value of all available information in order to make sound judgements and distinguish between good and bad potential leads. In the initial stages this may need to be done under time constraints. If so, the SIO should try to create some ‘slow time’ to consider these carefully and not act too hastily (investigative decision making is discussed in more detail in Chapter 2). The SIO needs to complete this task at the earliest opportunity so that the investigation is heading in the right direction, correct priorities are applied, and best use made of resources.

It is essential that the MLOE remain under constant review throughout the life of the investigation. There will always be changes—some will drop off the list once they have been dealt with or found to be not leading anywhere, or the investigation may change direction altogether. It is advisable to make regular reference to, and entries about, the MLOE in the SIO’s policy file, together with satisfactory rationale as to why some feature and others do not. The SIO must show responsibility and good control of the investigation and be confident about dropping or abandoning initial lines of enquiry.


Ensure that the main lines of enquiry listed have rationale recorded against each one in order to explain why they have been included. This should be recorded in the SIO’s policy file.

What the list should contain is entirely dependent on the individual circumstances of each case and investigation; this will differ from case to case. However, there is a good chance similar ‘main lines’ will appear regularly in most investigations. Here are some typical examples:

The list is a guide only and each case will require a unique list of MLOE, which may vary from week to week. Probably included

Checklist—MLOE examples

will be some quite specific main lines that the SIO feels strongly about, eg trace/interview Peter Sutcliffe potential significant witness who may have seen incident occur.

All the team must be kept fully aware of what is on the MLOE list and kept up to date with amendments or additions. The list should be displayed or kept in a prominent position for everyone on the enquiry to see and/or circulated to each and every team member. (Note: sensitive covert tactics may need to be left off the list for confidential reasons—if so, this can be explained with reasons and justification in a ‘sensitive policy file’ entry.)


When determining main lines of enquiry the SIO should have in mind para 3.4 of the Criminal Procedure and Investigations Act (CPIA) 1996, Code of Practice, which states that an investigator must pursue all reasonable lines of enquiry whether they point towards or away from the suspect.

About the Authors:

Detective Superintendent Tony Cook was a CID officer with Greater Manchester Police for over 31 years until his retirement in 2009. During his time as an SIO, he led a number of high profile investigations including operations into gangland violence at Moss Side, the Bolton murder of a teenage girl in 2002, and the Denton strangling case. He was a trained assessor for promotion and a qualified Authorising Officer under RIPA. Tony received 14 commendations and a first-class BSc Honours degree in social sciences and a Diploma in Social Policy & Criminology from the Open University.

Andy Tattersall, formerly Detective Superintendent in Greater Manchester Police on the Force Major Incident Team, retired in 2007 after 33 years service and became the first ever Support Staff SIO in charge of a new Homicide Support Unit. With over 29 years in CID at all ranks Andy received the Homicide Working Group National Award for his Outstanding Achievement in the Field of Homicide in November 2006.

To see more details about the Senior Investigating Officers' Handbook, or to purchase a copy, click here

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