The F.B.I. has arrested an Afghan citizen in Oklahoma City on charges of plotting a suicide attack on Election Day, with the intent of inflicting mass casualties on behalf of the Islamic State, according to a criminal complaint filed on Tuesday.

In preparation for the attack, the complaint said, the man, Nasir Ahmad Tawhedi, 27, liquidated his family’s financial assets to raise cash for the resettlement of his relatives in Kabul and recruited a co-conspirator, his nephew, who was not named because he is under the age of 18.

It is not clear where Mr. Tawhedi intended to stage the attack, though investigators said he planned to use two AK-47s. His online history showed that he searched for how to access cameras in Washington on the same day he visited the White House and Washington Monument webcams, according to investigators.

Mr. Tawhedi communicated his plans in chilling detail to a man he later identified as a member of the Islamic State, telling him he would obey any order he was given.