They are joining the Trayvon Martin crusade by the hour now.

It feels like an echo from another era — when there was racial injustice in the headlines, when federal troops were dispatched to comb Southern swamps to look for blacks who had vanished....

“It reminds you of Emmett Till,” said Bernadette Pruitt, an associate professor of history at Sam Houston State University in Huntsville, Tex., who has written about Southern racial history and can’t stop thinking of Trayvon Martin and his family. “This so-called post-racialism is a figment of our imagination. Race, unfortunately, is still the barometer by which everyone is measured.”

Investigating the killing of the 17-year-old Martin, who was black, by neighborhood watch captain George Zimmerman, who is Hispanic, is now a top priority for the FBI, senior law enforcement officials said Wednesday....