Reports in the Catalan media also say local authorities are looking into whether the event could have infringed restrictions in place in the region. People are allowed to meet in Catalonia, outside or inside, but only in groups of up to six.
Sources told ESPN that the players and their partners spent the afternoon outside and at separate tables, respecting social distancing regulations laid out by La Liga, but the Spanish league is seeking more information on the get-together.
Despite the success of the vaccination program in Europe, players must still follow strict guidelines on matchdays -- including getting changed alone before arriving at the stadium -- and adhere to testing and distancing rules put in place by the corresponding local government.
However, there is no precedent for players or clubs being punished for breaches relating to coronavirus regulations.
Four Sevilla players were criticised last May for attending a barbecue before the league restarted in June. La Liga president Javier Tebas said "measures would be taken" against them but the issue disappeared after the players issued public apologies.
A spokesman for Barca said they had not received any notification of an investigation from La Liga or local authorities and that it was an issue for the players to deal with.
Pere Aragones, the vice president of the Catalan government, called on those in the public eye like Barca's players to lead by example when asked about the incident in a news conference on Tuesday,
"At a time when we're taking steps forward to relax the restrictions, we must behave in an exemplary way, especially those with public profiles, who are an example for many people," Aragones said.
Messi, whose future remains unresolved with his contract expiring in June, gathered the squad at his house in Castelldefels, a seaside town outside of Barcelona, after Monday's training session ahead of what could prove a defining week in the title race.
With no game in midweek, coach Ronald Koeman had given the players Monday afternoon and Tuesday off. They don't train again until Wednesday evening.
They play league leaders Atletico at Camp Nou on Saturday knowing they will leapfrog them with a victory, although their hopes of winning the league also depend on Real Madrid, who host fourth-placed Sevilla on Sunday, dropping points.