WASHINGTON — The possible motivations of the two brothers linked to the Boston Marathon bombings are as yet publicly unknown. Of Chechen heritage, they lived in the United States for years, according to friends and relatives, and no direct ties have been publicly established with known Chechen terrorist or separatist groups.

Yet, with at least one brother talking of Chechen nationalism on the Internet, their reported involvement in the marathon attack throws a spotlight back on one of the darkest corners of nationalist and Islamic militancy, and to a campaign for separatism and vengeance responsible for some of the most unsparing terrorist acts of recent decades.

Fired by a potent mix of blood codes, separatist yearnings and Islamic militancy, Chechen groups have staged a string of intermittent but spectacular attacks in Moscow and elsewhere in Russia since the 1990s. They have bombed trains, planes and subways, attacked a rock concert and slammed a truck bomb into a hospital. In 2002, they seized a crowded theater in Moscow, an attack that culminated in a commando raid that killed 130 hostages.

In the spring of 2004, a bomb placed in a stadium in Grozny, the regional capital, killed the Kremlin’s handpicked Chechen president. That summer, female suicide bombers with hand grenades brought down two Russian passenger jets nearly simultaneously, killing 90 people.