Legitimate interest: the legal key that opens —or closes— every private investigation
Every week, someone calls a private investigation agency with a similar request: "I want to know everything about this person."
A professional investigator's answer should always be the same: "What for?" And, next: "On what grounds?"
Those two questions are not a formality. They are the legal starting point of every private investigation in Spain. Law 5/2014 calls it legitimate interest.
The law requires knowing why the investigation is taking place
Article 48.2 of the Private Security Act requires, in all cases, that the person making the request prove the legitimate interest they claim, and that this proof be included in the engagement and investigation file.
So when an agency asks about the relationship with the person to be investigated, the purpose of the assignment, or the documentation that justifies it, it is not making the job harder. It is complying with the law and protecting the evidence to come.
No prior assignment, no service
A professional investigation begins with a legitimately entitled client and a specific object. Article 25 of Law 5/2014 requires a written contract to be drawn up for each investigation service.
First there is a need. Then the right or interest that justifies it is identified. The object is defined. The assignment is formalised. And only then does the investigation begin.
A professional investigator does not gather information on their own initiative to sell it afterwards to whoever might be interested.
The interest is not presumed: it is demonstrated
Depending on the case, the proof can consist of a breached contract, a debt, a court ruling, an employment relationship, a non-compete agreement, a corporate relationship, a proceeding, a family conflict with legal consequences, or a right of credit.
There is no universal "legitimate interest certificate."
What exists is a legal reality capable of answering this question: what relationship exists between this client, this person under investigation, and these specific facts?
The investigator must also assess the assignment
The client does not unilaterally decide that their interest is legitimate and thereby oblige the investigator to carry out what was requested.
The agency must examine it. The Law treats it as a very serious infringement to carry out private investigations for clients in whom no legitimate interest is present.
Asking is not distrust. Asking is practising professionally.
Common scenarios of legitimate interest
There is no closed list, but in practice clear groups emerge.
Contractual relationships: breaches, competition, exclusivity, misuse of assets, or diversion of business activity.
Employment relationships: verification of facts relevant to compliance with employment obligations.
Corporate or commercial relationships: conflicts of interest, diversion of business, relevant transactions and economic relationships.
Family relationships with legal effects: maintenance, pensions, custody, assets, and compliance with court orders.
Exercise or defence of legal actions: when a person knows a fact but needs to obtain and document evidence in order to be able to prove it.
The STS 851/2021 case
A woman hired an investigator to obtain evidence about the professional and economic activity of her ex-husband, who had been failing to meet financial obligations arising from the divorce.
The person investigated sued the investigator and his ex-wife for infringement of his privacy and right to his own image.
The Supreme Court dismissed the claim: there was a legitimate interest, the investigation was suitable for obtaining information that was hard to get by other means, the actions respected spatial limits, and the reports had an evidentiary purpose.
The investigation was not legitimate because the investigator could freely investigate just anyone. It was legitimate because there was a legally justifiable relationship between the client's right, the facts investigated, and the means used.
Legitimate interest also defines how far the investigation can go
Legitimate interest does not only answer "can we begin?" It also answers "how far can we go?"
An investigation into unfair competition does not turn the entire life of the person investigated into the object of the investigation. Article 49 establishes that the report should only contain information directly related to the object and purpose of the assignment.
Legitimate interest opens the investigation, but it also draws its perimeter.
Legitimate interest and data protection
Every investigation normally involves the processing of personal data. Article 6(1)(f) GDPR provides for legitimate interest as one of the grounds that can legitimise processing when it is necessary and the rights and freedoms of the data subject do not prevail.
But the legitimate interest under the Private Security Act and the GDPR test should not be confused as if they were exactly the same thing. Data protection also requires purpose, necessity, minimisation, and proportionality, and certain categories of data may call for an additional analysis.
A professional investigation does not consist of gathering as much data as possible, but of obtaining the data necessary to prove what is being legitimately investigated.
Saying "no" is part of the service
There are assignments without sufficient legitimate interest. There are requests whose scope must be narrowed. There are clients seeking information they do not need in order to defend any right.
Saying "we can't do it that way" does not mean losing an assignment. It means protecting the client.
Because the aim of private investigation is not to get information at any price. It is to obtain usable evidence.
Conclusion
Every good investigation should be explainable in one sentence: "My client had the right to know and prove these facts because..."
Whatever comes after that "because" is the legitimate interest.
At Método 3 we ask two questions before we begin: what for? And on what grounds?
These are not the questions that hold a case back. They are the ones that let it begin properly.
Truth has a method.