A number of lawyers specialising in employment law, headed by Carlos del Río, had opened an office at No. 55, Calle Atocha. On 23 January 1977, various lawyers met with eighty representatives from the private transport industry. At the time, the sector was in the midst of an industrial dispute and there had been a number of dismissals. The meeting ended at around 10.30 pm, after which the lawyers stayed on at the office talking. At 10.45 pm, the bell rang and the porter, Ángel Rodríguez Leal went to open the door. Two people entered asking for the trade unionist Joaquín Navarro, who was in a nearby café. A third individual stayed at the door. The first two opened fire, fatally wounding Rodríguez Leal. They then mowed down the lawyers with machine guns, fitted with silencers. Two of the victims, Luis Javier Benavides Orgaz and Enrique Valdevira Ibáñez, were killed instantly; another two, Francisco Javier Sauquillo and law student Serafín Orgado, died later as a result of their injuries. Another four lawyers, Miguel Sarabia Gil, Alejandro Ruiz-Huerta, Luis Ramos and Lola González Ruiz, wife of Sauquillo, were injured.
The attack took place a year and three months after the death of Franco, at a time when the country was gripped by a wave of violence, with actions by both extreme-right wing groups and terrorist organisations such as GRAPO. Four people with links to the far-right were tried and sentenced: Fernando Lerdo de Tejada, who escaped while on parole; Carlos García Juliá and José Fernández Cerra, considered to be the perpetrators of the killing; and Francisco Corredera Albadalejo, who acted as an accessory to the crime.
Serafín Holgado, aged 27, had been born in Salamanca. He had only subject left to pass to complete his law degree. He combined his studies with a job at the Institute of Education Sciences. In late 1976 he got a place as an intern at the labour law firm in Atocha. He was an active member of the Communist Party.