In April 2012, the authorities said, an industrial mechanic walked into a synagogue in Albany and announced his intention to build a weapon that could help Israel kill its enemies while they slept. He wanted to know if anybody would provide financial backing. Turned away, prosecutors said, he sought money from another source: a leader in the Ku Klux Klan in North Carolina.

Both the synagogue and the white supremacist group told the authorities about the man, Glendon Scott Crawford, who, until his arrest this week, devoted himself to building a weapon of the sort he had promised, the authorities said. The weapon was an X-ray-emitting device that could be activated by remote control, which he intended to use to kill Muslims, according to a criminal complaint unsealed in Federal District Court in Albany.

Mr. Crawford, who the authorities say works for General Electric in Schenectady and lives in Galway, N.Y., believed the device would enable him to secretly poison people with lethal doses of radiation from a safe distance, the authorities said. On Wednesday, federal prosecutors charged Mr. Crawford, 49, and an engineer, Eric J. Feight, 54, of Hudson, N.Y., whom the authorities described as a co-conspirator who works in industrial automation, with conspiring to provide support for the building of a weapon of mass destruction. The authorities say Mr. Crawford relied on Mr. Feight to design the weapon.

Mr. Crawford, the authorities said, conceived of a powerful X-ray device that could be placed in a truck and driven near a target. The driver would park, leave the area and activate the device, “killing human targets silently and from a distance with lethal doses of radiation,” the complaint against the men stated.