McLaren's grand plans to conquer the Indianapolis 500 and the world of Indycar more generally are in tatters.

Fernando Alonso quit F1 last year, but together with McLaren he returned to Indianapolis for 2019 to complete his 'triple crown' project.

However, on Sunday, the McLaren-Alonso entry failed merely to qualify for the Indy 500 following a crash, technical problems and a lack of performance.

In response, McLaren supremo Zak Brown promptly fired former Force India boss Bob Fernley, who was running the brand's Indy project.

"I don't think we underestimated Indy, but I think we were unprepared," Brown told the Indianapolis Star newspaper.

Alonso says his orange car was "slow", and not just in terms of performance.

"You see Juncos (Racing) crashing yesterday and being ready at 6am. For us, we've been a little bit slow on everything," the Spaniard added.

One interpretation is that Brown was blaming Fernley and technical partner Carlin Racing for many of the problems.

But Brown insists: "McLaren is fully responsible for our lack of success."

However, he added that "two of their (Carlin's) three cars" failed to qualify for the Indy 500.

Nevertheless, Brown says the 2019 Indy 500 disaster has not dampened his desire to see McLaren succeed in America.

"While this is obviously a massive event and a very public disappointing result, we're a racing team," he said.

"In racing, you win races, you loses races, you crash. That's all part of it.

"McLaren has a lot more resolve than letting a race result shy us away from coming back or pushing through something we want to do, just because we lost."