Why wasn't Cedric Diggory's death investigated?

Question

Cedric's dead body was found at the edge of the maze constructed for the third task of the Triwizard tournament, with Harry lying by his side clutching onto his arm.

He was killed by Peter Pettigrew using Avada Kedavra.

While we know that Fudge (and hence the Ministry of Magic) does not believe Voldemort is back, there is no mention of any sort of investigation into Cedric's death. We hear Umbridge briefly mention it in OotP:

“So, according to you, Cedric Diggory dropped dead of his own accord, did he?” Harry asked, his voice shaking.
...
“Cedric Diggory’s death was a tragic accident,” she[Umbridge] said coldly.
(brackets my own)
_{Harry Potter and the Order of the Pheonix, Chapter 12: Professor Umbridge}

We can assume "tragic accident" is the official position of the Ministry regarding Cedric's death. Surely, the circumstances surrounding his death were highly suspicious and hence demanded an investigation. I'm sure there would be experts in the Ministry who could have easily figured out that he was, in fact, killed by the Avada Kedavra curse. At the very least, why wasn't Harry questioned about it? He was the one found clutching Cedric's dead body. My question is

How did the Ministry come to the conclusion that Cedric's death was a tragic accident?

I'd appreciate answers from canon.

Answer 1

Given that Cedric was participating in the TriWizard Tournament (which was known for killing its participants) and the Ministry didn't want to even hint that Voldemort may be back, it is likely that his death was covered up completely. All anyone outside of the Great Hall would have heard from 'official sources' would be that he died in the last challenge of the Tournament, possibly with a description of the maze and its many tricks, traps, and monsters.

It is likely that any investigation was a complete whitewash, where Ministry officials provided Fudge with the results he wanted in very short timeframes with minimal real study or investigation.

Fudge decided that there was no way Cedric could have been killed by Voldemort. He then pressured his underlings to support this story, and likely assigned his most loyal followers to 'look into' the matter. They would have done so, and since 'qualified people' were already looking into it, Fudge would have blocked any alternative investigations as being wasteful of Ministry resources.

That's how corrupt governments roll.

Answer 2

Fudge and the Ministry

Fudge refused to believe something he did not want to be true. Of course, Fudge had an explanation for himself to believe in, that Barty Crouch jr. was acting alone. This allowed him to believe that while some awful events had occurred, everything had been resolved and general order hadn't been upset.

"Why he killed them? Well, that's no mystery, is it?" blustered Fudge. "He was a raving lunatic! From what Minerva and Severus have told me, he seems to have thought he was doing it all on You- Know-Who's instructions!"
"Lord Voldemort was giving him instructions, Cornelius," Dumbledore said. "[...] Voldemort has been restored to his body."
Fudge looked as though someone had just swung a heavy weight into his face. Dazed and blinking, he stared back at Dumbledore as if he couldn't quite believe what he had just heard. He began to sputter, still goggling at Dumbledore. "You-Know-Who . . . returned? Preposterous. Come now, Dumbledore . . ."

[...]

"You fool!" Professor McGonagall cried. "Cedric Diggory! Mr. Crouch! These deaths were not the random work of a lunatic!"
"I see no evidence to the contrary!" shouted Fudge, now matching her anger, his face purpling. "It seems to me that you are all determined to start a panic that will destabilize everything we have worked for these last thirteen years!"

[...]

"You are blinded," said Dumbledore, his voice rising now, the aura of power around him palpable, his eyes blazing once more, "by the love of the office you hold, Cornelius! [...]"

_{Harry Potter and the Goblet of Fire, chapter 36, "The Parting of the Ways"}

Fudge did not want Voldemort to have returned, so he refused to believe that had happened.

It seemed Fudge could think of no answer to this. He rocked backward and forward on his small feet for a moment and spun his bowler hat in his hands. Finally, he said, with a hint of a plea in his voice, "He can't be back, Dumbledore, he just can't be . . ."

_same

The Wizarding World

So Fudge had an explanation for himself to cling to. He also had an explanation for the wizarding world in general.

The Triwizard Tournament was known to be dangerous, deadly even. This was even deemed acceptable until the death toll mounted too high.

"The Triwizard Tournament was first established some seven hundred years ago as a friendly competition between the three largest European schools of wizardry: Hogwarts, Beauxbatons, and Durmstrang. A champion was selected to represent each school, and the three champions competed in three magical tasks. The schools took it in turns to host the tournament once every five years, and it was generally agreed to be a most excellent way of establishing ties between young witches and wizards of different nationalities — until, that is, the death toll mounted so high that the tournament was discontinued."

_{Dumbledore explaining the Triwizard Tournament in Harry Potter and the Goblet of Fire, chapter 12, "The Triwizard Tournament"; emphasis mine.}

Of course, we're now centuries later.

"There have been several attempts over the centuries to reinstate the tournament," Dumbledore continued [...]

_same

But it is still dangerous.

"[...] the tournament tasks will still be difficult and dangerous, whatever precautions we take [...]"

_same

All in all, Cedric's death could be chalked up to his participation in a dangerous tournament that has been known to cause deaths in its previous instalments.