Read a joint statement about this story from the Guardian and Televisa here.

The Guardian has had access to dozens of digital files that appear to have originated at a marketing company, Radar Servicios Especializados (RSE), that suggest the Mexican television network Televisa sold news coverage and entertainment content to political clients. The bulk of the collection is made up of financial spreadsheets, presentations of proposals and a few other related documents. The files also include personal letters, which the Guardian has not published. The Guardian’s source previously worked with Televisa.

What steps has the Guardian taken to verify the documents?

It has not been possible to confirm the authenticity of the documents beyond doubt, but the Guardian has made extensive efforts to find coroborating evidence. The dates on which the documents were created and modified, and the names under which they are saved, correspond to the events they refer to. The Guardian spoke to a second source who had previously worked for Televisa and confirmed that meetings had taken place within the company to discuss a campaign to damage the presidential chances of Andrés Manuel López Obrador. The source – who has no personal sympathy for the former Mexico City mayor – said the sale of political content was widespread, particularly oblique promotion of political clients within entertainment shows. The Guardian also spoke to a politician who described how on one occasion Televisa offered to develop a sympathetic storyline in a soap opera adapted to their needs.

Could they be forgeries?

Without forensic tools that are available only to law enforcement agencies, it is impossible to say where and when any digital document has been created, but the quantity and variety of the documents and presence of personal material mixed in with the work documents suggests authenticity.

Did the actions proposed or described in the documents actually take place?

Many of the actions did:

• The anti-López Obrador presentation proposes asking the then president, Vicente Fox, to declare a period of national mourning following the death of Pope John Paul II. Hours after the document was apparently drawn up, Fox declared a day of mourning.

• The Guardian has confirmed that some Televisa celebrities publicly recounted personal stories of crime in the capital, as suggested in the proposal. The presenter of one show mentioned in the document went public with a story about his wife being followed in a Mexico City shopping centre. He was subsequently interviewed on radio by one of two Televisa celebrity journalists specifically named in the document as to be instructed on the editorial line.

• Personal crime stories were also featured within at least one episode of Big Brother VIP, Mexico’s celebrity version of the reality show.

• Televisa regularly revisited old corruption scandals involving López Obrador’s former collaborators, as proposed.

Has the Guardian verified the actions described in one document that details work done for Fox’s presidential office?

The Guardian confirmed that at least three interviews referred to in the document did take place. Since publication, it has found evidence in public accounts that the presidential office did pay RSE 3m pesos (£140,000) in 2005 as stated in the document.

Did the Guardian give Televisa the right to reply?

The Guardian’s reporter went to Televisa several days before publication in order to give the company the opportunity to respond to the allegations, to raise specific questions about whether the network had ever sold any kind of coverage to political clients, and to request clarification on Televisa’s relationship with RSE. The company declined the opportunity to respond without first seeing the files, which were originally handed to the Guardian in confidence. Our source has now agreed to let us publish them and they appear online here.

Did the Guardian give the politicians named in the documents the right to reply?

Our reporter contacted Enrique Peña Nieto’s team several days before publication and later detailed the allegations relating to him in an email, along with specific questions about whether the presidential favourite ever paid for coverage when he was governor of the state of Mexico. A spokesman for the candidate sent a written response in which he stated that the team had not seen the documents and denied the existence of such “contracts”. The Guardian has never referred to the documents as contracts.

The Guardian also sought responses from the other politicians mentioned in the documents, including Fox. All declined to be interviewed except the former senator Demetrio Sodi, whose response was included in the coverage.

Have the documents been published before in Mexican media?

One of the documents seen by the Guardian appears to be a digital version of a printed budget proposal for Peña Nieto that was first published in 2005 in the weekly news magazine Proceso. The Guardian’s source did not provide Proceso with the document.

Why does the Guardian feel justified in publishing the documents?

There is a clear public interest to investigate allegations of media bias in Mexico, where the issue has become a central theme of the current presidential election campaign. Media experts in Mexico have long pointed out a serious problem of transparency in public spending on political propaganda.