The Ra'zac destroyed the house.
Eragon noticed two sets of footprints, consistent with two Ra'zac:
He focused on the trampled snow. The faint impressions of two pairs of leather boots headed toward the house. On top of those were traces of the same two sets of boots leaving
He does not notice any monster tracks from the assailants. Lethrblaka make very obvious marks:
Numerous Ra'zac footprints overlapped each other. Eragon guessed that the tracks were only a few days old. Superimposed over them were long, thick gouges torn in the ground. They looked familiar, but Eragon could not say why. He stood shaking his head. "I don't have any idea what..." Then his eyes fell on Saphira and he realized what had made the gouges. Every time she took off, her back claws dug into the ground and ripped it in the same manner.
It seems unlikely he missed similar marks at the farmhouse. The tracks that the villagers found would then be Saphira's:
Eragon ran to the wreckage, hunting through the destroyed rooms for Garrow. There was no sign of him. "Uncle!" Eragon cried again. Saphira walked around the house and came to his side.
"Saphira! I need you!"
She came immediately. Wood cracked under her feet as she crawled over the ruined walls. Without a word she nosed past him and set her side against the beam.
Saphira confirmed as much later:
I don't care what you say; those aren't reasons to leave! cried Eragon
Then here are others. My tracks have been seen, and people are alerted to my presence.
Eragon also overheard Horst discussing the site where Saphira landed after carrying Garrow, so we can rule out that Saphira and the Lethrblaka both made prints:
"When we started for their farm, the road was scraped smooth by the board he dragged Garrow on. Then we reached a place where the snow was all trampled and churned up. His footprints and the signs of the board stopped there, but we also saw the same giant tracks from the farm."
There's not any evidence that the Lethrblaka were in the area. Eragon tracked the Ra'zac footprints for days before finding their aforementioned marks, and the Raz'ac felt the need to get horses in Therinsford rather than flying, despite their haste:
"The Ra'zac definitely passed this way. Apparently they stopped here to pick up horses, as we did. I was able to find a man who saw them. He described them with many shudders and said that they galloped out of Therinsford like demons fleeing a holy man."
That the house was "torn to pieces" is Horst's interpretation; from Eragon's viewpoint we see that it was more consistent with an explosion:
The house had been blasted apart. Timbers and boards that had been walls and roof were strewn across a wide area. The wood was pulverized, as if a giant hammer had smashed it. Sooty shingles lay everywhere. A few twisted metal plates were all that remained of the stove. The snow was perforated with smashed white crockery and chunks of bricks from the chimney. Thick, oily smoke billowed from the barn, which burned fiercely.
He even described the same to Horst:
"Nothing... nothing happened that night. The next morning I finished my chores and went walking in the forest. Before long I heard an explosion and saw smoke above the trees. I rushed back as fast as I could, but whoever did it was already gone.
The Ra'zac are shown to use explosives in Eldest:
The creature was illuminated by a torch in its left hand, while its right was drawn back, as if to throw something.
Roran laughed. "Is he going to toss rocks at us? He's too far away to even hit-" He was cut off as the Ra'zac whipped down its arm and a glass vial arched across the distance between them and shattered against the wagon to his right. An instant later, a fireball launched the wagon into the air while a fist of burning air flung Roran against the wall.
Incidentally, these explosives are either chemical in nature or a potion supplied by someone else, as Oromis tells us the Ra'zac are not magicians:
"...Their greatest weapon is their evil breath, which fogs the minds of humans-incapacitating many ... Though the Ra'zac cannot use magic, they are not to be underestimated."
Eragon also found Seithr oil the Ra'zac left behind:
Just as he was about to enter the trees, his foot struck something hard. Lying on the ground was a metal flask with a leather strap just long enough to hang off someone's shoulder. A silver insignia Eragon recognized as the Ra'zac's symbol was wrought into it.
Excited, he picked up the flask and unscrewed its cap. A cloying smell filled the air-the same one he had noticed when he found Garrow in the wreckage of their house.
...Eragon remembered the terrible burns that had covered Garrow. That's what they used on him, he realized with horror.
The same passage then shows that Galbatorix was not in the area, at least to Brom's knowledge:
"But why didn't [the Ra'zac] come back for it? I doubt that the king will be pleased that they lost it."
"No, he won't," said Brom, "but he would be even more displeased if they delayed bringing him news of you.
Galbatorix doesn't search for dragons himself, as Brom explained earlier:
You see, they are the king's personal dragon hunters. Whenever rumors reach Galbatorix of a dragon in the land, he sends the Ra'zac to investigate. A trail of death often follows them.
Finally, Garrow was holding a scrap of cloth that matched the Ra'zac's outfits:
As he did, a scrap of black cloth fell from his uncle's hand. It matched the strangers' clothing.
The Ra'zac are in the area, searching for Saphira, are capable of the destruction without the help of Shruikan or the Lethrblaka, and had Seithr oil, which was used in the attack. All the evidence points to the same conclusion several characters make:
The next morning Eragon avoided bringing to mind any of the recent events; they were too painful for him to consider. Instead, he focused his energies on figuring out how to find and kill the Ra'zac. I'll do it with my bow, he decided, imagining how the cloaked figures would look with arrows sticking out of them...In a few days, maybe a week, he would use his arrows to avenge Garrow's death.
Baldor abruptly spoke. "I don't like this. Too much of this rings of wizardry. Who are these men? Are they Shades? Why did they want the stone, and how could they have destroyed the house except with dark powers?"
When he mentioned the strangers, Saphira recoiled. She reared and rored deafeningly, then whipped he tail over his head. He scrambled back in surprise, ducking as her tail hit a snowdrift. Bloodlust and fear emanated from her in great sickening waves. Fire! Enemies! Death! Murderers!...
Eragon dragged Garrow out of the destroyed house... A cloying, sickening smell hung over him-the odor of rotting fruit. His breath came in short jerks, each one sounding like a death rattle.
Murderers, hissed Saphira.
[Brom] took a deep pull on the pipe. "I was sneaking around the Ra'zac's camp after dark, trying to learn what I could, when they surprised me in the shadows... Stunned, I fell to the ground and didn't regain consciousness until the next day. By then they had already arrived at your farm."
But now that Oromis's explanation had stripped away the Ra'zac's aura of mystery, they no longer seemed quite so formidable. The fact that they were vulnerable to light and water strengthened Eragon's conviction that when they next met, he would destroy the monsters that had killed Garrow and Brom.