From the New York Times obituary:
The two were together when Till allegedly whistled at a white woman, Carolyn Bryant, at the convenience store she owned with her husband, Roy. Wright said Till was “always joking around” and was likely trying to get a laugh out of his cousins. But the whistle struck Wright, who feared the overwhelming presence of the Ku Klux Klan in Mississippi, to the core. Chicago Magazine quoted Wright as saying the joke “scared us half to death … A black boy whistling at a white woman? In Mississippi? No.”
The group promised not to tell Wright’s father about the incident, expecting that he would rush Till out of town if he ever found out.
But it was at 2 a.m. on Aug. 28 that Roy Bryant and his half brother, J.W. Milam, arrived at the Wrights’ home. They snatched Till from the bed he shared with Wright. Till’s beaten body was later found in the Tallahatchie River, along with a 75-pound cotton-gin fan tied to his neck with barbed wire.
The accompanying National Archives video features an interview with Simeon Wright.