Allyson Felix and Jeneba Tarmoh are waiting to hear if they will face a run-off for a place on the US Olympic team.
The pair dead-heated for third place in the 100m at the National Championships in Eugene, Oregon - which double as the Olympic trials.
An initial announcement to the crowd suggested Tarmoh had been given the verdict by 0.0001 seconds before the results were then reviewed.
And with no tie-break procedure included in USA Track and Field's regulations, the governing body is now considering their options.
Felix and Tarmoh are likely to be given the choice of a run-off or a coin toss. Should they disagree on their preferred method then a run-off will be used, but if neither declares a preference then a coin-toss will be used.
In the other action at Hayward Field, Justin Gatlin ran 9.80 seconds to win the men's 100m and qualify for his second Olympics along with Tyson Gay (9.86) and Ryan Bailey (9.93).
LaShawn Merritt - who like Gatlin has returned from a drug ban to earn selection - ran a world-leading 44.12s to win the men's 400m. Tony McQuay and Bryshon Nellum ran personal bests to join Merritt on the Olympic team.
Sanya Richards-Ross secured her place in the women's event in even more emphatic fashion. Her time of 49.28s was also the best in the world this year and gave her a winning margin of 0.74sec ahead of fellow qualifiers DeeDee Trotter and Francena McCorory.
Marquise Goodwin produced a PB of 8.33m to defend his national title in the men's long jump and qualify first ahead of Will Claye and George Kitchens Jr.
Suzy Powell-Roos misses out on the a place in the women's discus despite finishing third behind Stephanie Brown-Trafton and Aretha Thurmond. Powell-Roos has failed to attain the A standard, so sixth-placed Gia Lewis-Smallwood will head to London instead.
Reese Hoffa, who set a world-leading mark, Ryan Whiting and Christian Cantwell took the three places in the men's shot put while 2008 silver medallist Jenn Suhr was joined in the women's pole vault squad by Becky Holliday and Lacy Janson.
