The office of Veracruz state Gov. Javier Duarte said the victims were Mexican tourists from Texcoco, a city in Mexico state.

The mother, 38, and their children — a boy and a girl younger than 6 — were in intensive care. The state Public Health Department said the children needed surgery.

They were felled by an explosion that shattered the calm of a Sunday outing on a thoroughfare leading to the city's beach. The boulevard is lined with restaurants and shops selling beach wear and was crowded with strollers.

The government statement said gunmen fleeing police tossed the grenade from their car, apparently to stop police from pursuing them, a tactic that has been used in the past by Mexico's drug gangs.

The National Human Rights Commission said Monday that it is launching an investigation into the attack as part of its role in defending victims' rights.

Mexico's drug gangs sometimes use hand grenades to attack police and civilians, and such gangs have been active in Veracruz