World indoor silver medallist Ajee Wilson was leading the race on the final bend when Martinez, in second place, began to stumble and Montano, in fourth, tripped over her feet.

Seven-times US champion Montano, who runs with a flower in her hair, was scythed to the ground, while Martinez stumbled out of contention. 

The two women expected to lead the field home finished second last and last.

Montano, who notably ran in 2014 while eight months pregnant and who had won seven previous national titles, was stunned afterwards.

“It was incidental contact from Brenda, It was like a domino effect, there was nothing you could do about it," she said.

"I had so much power and strength. 

"I was in the best shape of my life. 

"My heart is in a million pieces.”

Wilson held on to take second place in 1min 59.51sec with Kate Grace coming through late to win in 1:59.10. 

The third Rio spot went to Chrishuna Williams in 1:59.59.

Having won the world indoor title in Oregon earlier this year, Boris Berian returned to the podium in Eugene with second place in the men’s 800m.

Berian, freed recently from his wrangle with Nike over a contract, clocked 1:44.92 behind Clyaton Murphy, who won in 1:44.76.

Charles Jock took third place in 1:45.48.

Sam Kendricks won the pole vault competition as expected, seeing off all opposition with first time clearances up to 5.70m and then progressing to a meeting record of 5.91m, just a centimetre shy of his personal best.

The US Army second lieutenant then had an unsuccessful shot at six metres.

Second place went to US Air Force’s Cale Simmons, who cleared 5.65m, with the third qualifying spot being claimed by Logan Cunningham with a best of 5.60m.

Cyrus Hostetler won the men’s javelin with a best of 83.24m, with Curtis Thompson, with 82.88m, and Riley Dolezal, with 79.67m, claiming the next two places.