A shadowy group called the Chinese Martyrs’ Brigade has claimed responsibility for the disappearance of Malaysia Airlines Flight MH370 — but officials were skeptical and said the claim could be a hoax.

The group — unheard of before now — on Sunday sent an email to journalists across China that read: “You kill one of our clan, we will kill 100 of you as pay back,” but the message provided no details of what brought the flight down.

And Malaysia’s acting transport minister, Datuk Seri Hishammuddin Hussein, told reporters at Kuala Lumpur International Airport on Monday he doubted the claim’s legitimacy.

“There is no sound or credible grounds to justify their claims,” he said, according to Malaysian news reports.

Other officials said the claim could be a hoax aimed at increasing ethnic tensions between Uighurs and Han Chinese in the wake of the recent knife attack in the southwestern city of Kunming on March 1 that left 29 people dead and injured about 140 others.

The message was delivered through an anonymous, encrypted Hushmail service that is virtually impossible to trace, they said.

Investigators also said Monday that debris spotted from the air that was originally believed to be from the plane turned out to be a large cable spool unconnected to the aircraft.

Officials also said testing showed that an oil slick thought to possibly be from the plane had no connection to the aircraft.

Meanwhile, investigators suspect the vanished Malaysian airliner may have been blown out of the sky — just like the jumbo jet that rained deadly wreckage onto Lockerbie, Scotland, in 1988.

A senior official involved in the probe of its disappearance said the evidence so far “appears to indicate that the aircraft is likely to have disintegrated at around 35,000 feet,” Reuters reported.

Asked if that suggested a bomb blew up the Boeing 777, the source said there was no evidence yet of foul play, but noted the closest parallels to the plane’s disappearance early Saturday over the South China Sea were the 1980s bombings of Pan Am Flight 103 over Lockerbie and Air India Flight 182 off the coast of Ireland.

Although the source added that the flight, en route from Kuala Lumpur to Beijing, could have broken apart due to mechanical failure, Malaysian officials have not ruled out a hijacking.

Meanwhile, Hussein said authorities have surveillance video of the two passengers who boarded the plane using stolen passports.

Malaysia’s Civil Aviation Chief Azaharuddin Abdul Rahman said officials had reviewed surveillance tape of the plane’s boarding and are now saying the pair were not Asian, as they had originally indicated.

“We confirmed now they are not Asian-looking males,” Rahman said, adding that one of the men was black.

One had been identified, officials said, though they refused to release a name or nationality.

Five booked passengers failed to show up for the flight, according to The Wall Street Journal.

Other troubling details emerged when a telephone operator on a China-based hot line for KLM airlines confirmed Sunday that the passengers traveling with the stolen passports had booked one-way tickets on the same KLM flight from Beijing to Amsterdam on Saturday.

The operator told the Associated Press that they had booked the tickets through China Southern Airlines, but she had no information on where they bought them.

Interpol — which called the use of the stolen passports a “great concern” — said a check of all documents presented by passengers on the ill-fated flight also revealed additional “suspect passports.”

An Interpol spokeswoman couldn’t say how many other passports were being investigated, or what country or countries issued them.

Malaysia’s air force chief, Rodzali Daud, said radar showed the plane — which carried 239 people — may have turned back toward Kuala Lumpur.

No distress signals from the crew were reported.