Sandinista supporters came under fire as they returned from celebrations to mark the 35th anniversary of Nicaragua's rebellion to overthrow dictator Anastesio Somoza.
Five people died.
Another 24 were wounded in Sunday's attack less than 100 kilometres north of the capital.
Authorities aren't publicly identifying any suspects.
(SOUNDBITE)(Spanish) AMINA GRANERA, NICARAGUAN POLICE CHIEF, SAYING:
"We've already located a criminal group which was responsible for the San Ramon massacre. For obvious reasons we're not going to name them. We're pursuing them. We're talking about a group of murderers. In the same way we're seeking to identify and capture several, no more than four, who were responsible for the massacre. We can't give more details."
An anti-Sandinista group is claiming responsibility for the attack on Facebook.
