Why did the Death Eaters expect the Longbottoms to know where Voldemort was?

Question

The Lestranges and Barty Crouch Junior tracked down Frank Longbottom in the belief that he knew the "present whereabouts" of Voldemort.

"We have heard the evidence against you. The four of you stand accused of capturing an Auror - Frank Longbottom - and subjecting him to the Cruciatus Curse, believing him to have knowledge of the present whereabouts of your exiled master, He Who Must Not Be Named -"
"Father, I didn't!" shrieked the boy in chains below. "I didn't, I swear it, Father, don't send me back to the Dementors -"
"You are further accused," bellowed Mr Crouch, "of using the Cruciatus Curse on Frank Longbottom's wife, when he would not give you information. You planned to restore He Who Must Not Be Named to power, and to resume the lives of violence you presumably led when he was strong."
_{(Goblet of Fire, Chapter 30, The Pensieve).}

Why on earth would Frank Longbottom be expected to know the current whereabouts of Lord Voldemort? Presumably, if he had have known then he would've tracked Voldemort down himself with a group of Aurors. It seems bizarre that Longbottom should be singled out as this great source of potential intel. Why was Longbottom picked over and above everybody else? Did it have anything to do with the prophecy?

I guess the overarching question is: did Frank Longbottom actually have any information that was any use to them (because if he did that would explain why they went after him)? Did he lose his sanity as a hero, denying them a crucial piece of information that would actually help them? Or did he go down because of he genuinely couldn't tell them what they were trying to find out?

Answer 1

Their reason was likely that the Longbottoms were skilled Aurors.

The Death Eaters don’t seem to have any specific reason to have thought the Longbottoms would know what had happened to the Dark Lord or where he was. J.K. Rowling says the reason that Bellatrix tortured them so wrathfully was purely due to their efficiency as Aurors.

SU: Oh, that's so cool. Can we just ask kind of a sad thing, though. What did the Longbottoms do that they earned that wrath from Bellatrix? Such-- There's three times, like the Potters thrice defied the Dark Lord.

JKR: They were efficient! They were efficient. That's all they needed to do to earn her wrath. They were-- They had rounded up Death Eaters, they were very good Aurors, they knew what they were doing, they were responsible for a lot of captures and arrests and imprisonments. And-- So there you are.
_{^{- PotterCast Interviews J.K. Rowling, part 1 (December 17, 2007)}}

From this, it seems likely that the Death Eaters were taking a chance and hoping that maybe the Longbottoms might know something about the Dark Lord’s whereabouts, and even if they didn’t, the Death Eaters would get to torture some of their stronger opponents. As Augusta Longbottom says, the two of them were highly gifted and well-respected in the wizarding community.

“They were Aurors, you know, and very well respected within the wizarding community,’ Mrs Longbottom went on. ‘Highly gifted, the pair of them.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 23 (Christmas on the Closed Ward)}}

The Death Eaters generally enjoyed torturing opponents of the Dark Lord, and the Longbottoms would be no different - as opponents of the Dark Lord who captured Death Eaters, they’d be prime targets even without the possibility that they might have information on the Dark Lord.

“There was Antonin Dolohov,’ he said. ‘I – I saw him torture countless Muggles and – and non-supporters of the Dark Lord.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 30 (The Pensieve)}}

It seems like their torture of the Longbottoms was simply in the hope they might have information, and also likely somewhat because they were angry at the Dark Lord’s defeat, and torturing Aurors was at least something they could try in their attempt to find him

The Lestranges didn’t know about the prophecy or Neville’s role.

Though the Longbottoms’ son Neville was one of two possible people that the prophecy said would have the power to defeat the Dark Lord, J.K. Rowling confirmed on her website that the Lestranges were targeting Neville’s parents, and didn’t know about the prophecy.

The Lestranges were sent after Neville to kill him

No, they weren’t, they were very definitely sent after Neville’s parents. I can’t say too much about this because it touches too closely on the prophecy and how many people knew about it, but the Lestranges were not in on the secret.
_{^{- The Lestranges were sent after Neville to kill him, Rumors (J.K. Rowling’s website)}}

Since they didn’t know about it, the prophecy couldn’t have been part of their decision to go after the Longbottoms to torture them for information on the Dark Lord.

Answer 2

When Voldemort disappeared just after the attack on Potters the death eaters were in total disarray. Some believed Voldemort was truly gone and totally abandoned the rest. Some were caught and in order to save themselves gave up some of the other death eaters. Others still went ahead and claimed to be under Imperius Curse and went on with their lives. Few of the death eaters like Bellatrix and Barty Crouch Jr. were his most loyal (and slightly deranged) followers. To them Voldemort was like God and they worshipped the very ground on which he walked and sought out any and every method on tracking him.

So I don't think the Longbottoms were targeted because they had knowledge about Voldemort's whereabouts. The few reasons they could have selected the Longbottoms may be:

• The Longbottoms were very popular as Dubmbledore himself said in the books:

“The Longbottoms were very popular,” said Dumbledore. “The attacks on them came after Voldemort’s fall from power, just when everyone thought they were safe. Those attacks caused a wave of fury such as I have never known.

Harry Potter and the Goblet of Fire, The Pensieve.

• They were one of the top members of Order of the Phoenix.

• They had already defeated Voldemort three times.

"The one with the power to vanquish the Dark Lord approaches... Born to those who have thrice defied him, born as the seventh month dies... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not... and either must die at the hand of the other for neither can live while the other survives... The one with the power to vanquish the Dark Lord will be born as the seventh month dies..."

The odd thing, Harry, he said softly, is that it may not have meant you at all. Sybillʹs prophecy could have applied to two wizard boys, both born at the end of July that year, both of whom had parents in the Order of the Phoenix, both sets of parents having narrowly escaped Voldemort three times. One, of course, was you. The other was Neville Longbottom.

Order of the Phoenix :- The Lost Prophecy

I don't think Bellatrix and the others knew anything about the Longbottoms being related to the Prophecy or even what the contents of the Prophecy was. Voldemort, who was highly secretive, worked alone and didn't trust anyone would never have confided the contents of the Prophecy to his death eaters.