A young man looks at a photograph of his 23-year-old brother, Ali Kareem, who went missing from Camp Speicher in northern Iraq in June. (Haider Hamdani/For The Washington Post)

As Iraq hunkers down in its war against Islamist extremists, bodies stream back to the Shiite heartlands that provide an increasingly critical supply of fighters. But the coffins that don’t return are also causing fury.

In Nasiriyah, an impoverished dust bowl of bare cinder-block buildings about 225 miles southeast of Baghdad, the governor estimates that more than 400 soldiers from the city are missing in action in the wake of a sweep by Sunni extremists across the country’s north.

Most are presumed killed in a single incident: the execution in June of as many as 1,700 soldiers from Camp Speicher, a former American military base outside the city of Tikrit that was under Iraqi control before being overrun by Islamic State militants in June.

On Tuesday, about 100 Iraqis stormed the country’s parliament in Baghdad, demanding answers about their missing relatives, officials said. Speaker Salim al-Jubouri promised to hold a parliamentary session on the issue Wednesday.

In towns and villages in Iraq’s Shiite south, families have been clamoring for information and holding protests to demand the bodies of their loved ones.

Dakhil Hammadi Issa and his wife, Kamila, hold a picture of their 24-year-old son, who has been missing since June. (Haider Hamdani/For The Washington Post)

They have threatened reprisal killings against Sunnis if bodies of their relatives are not returned and the killers are not held accountable, underscoring the intensifying fractures a new Iraqi government must repair if the country is to hold together. Iraq has been torn apart by waves of bombings and revenge killings related to sectarian tensions unleashed after the 2003 U.S.-led invasion.

“There is so much anger, and unless the government responds there will be major chaos in the south,” said Ali Abed al-Baderi, a 33-year-old Nasiriyah resident whose brother has been missing since June. “We have Sunni people here, and we are going to slaughter them. It’s not going to help, but at least we’ll get our revenge.”

Snippets of what may have happened at Speicher emerged when Islamic State militants released photos they claimed showed a mass execution of soldiers. A handful of survivors also brought tales of horror, though also a glimmer of hope that others may have escaped.

For a few families, dreadful closure came last month as Islamic State extremists made public a video to mark the Muslim festival of Eid al-Fitr at the end of the holy month of Ramadan.

The 36-minute video ends with a graphic killing spree.

When Sabah Kadhim Sayhood, 35, and his family gathered to watch, he spotted his 27-year-old brother cowering in the back of a cattle truck before lying facedown in the dirt and being shot by militants toting Kalashnikov rifles.

“All of us — my mother, the four children — all of us were watching,” Sayhood said. “This was our first day of Eid. You can’t imagine the tragedy.”

As well as the return of the bodies, family members are demanding the prosecution of army officers at the Speicher base for not better protecting the facility. “We want them to cut the heads of these leaders,” said Sayhood, whose family held a funeral for his brother after the video was released, despite not having the body.

Prime Minister Nouri al-Maliki last week announced a committee to investigate the Speicher deaths. “The Iraqi parliament still has no accurate information about what happened,” Ahmed al-Jubouri, a spokesman for the speaker of parliament, said Tuesday.

Nasiriyah is used to paying a heavy price for Iraq’s conflicts.

One of its poorest pockets is known as the Martyrs’ District because of the death toll here during the 1980-1988 Iran-Iraq war. Rubbish is strewn across the sunbaked dirt streets in the country’s fourth-largest city.

“In this neighborhood there used to be so many young people,” said 19-year-old Ihab Ismail, whose 27-year-old brother, Mustafa, was presumed killed in a helicopter crash in July.

Despite receiving a notice of death from the military, the family refuses to hold a funeral until his remains are returned. Without his body, his wife still holds out hope that he was captured, not killed, when his helicopter went down in the western Anbar province, where the Islamic State controls large tracts of territory.

Families from across the country have traveled the grim circuit of hospitals and morgues looking for information on the missing. Many say they are turned away with little assistance.

At the central Baghdad morgue, some bodies suspected to be from mass killings have turned up.

“These are all from Speicher,” said Ammar Ridha, a worker at the morgue, opening a grimy refrigerator to show some of the 116 bodies that he said arrived after floating downstream. Now they are piled up in black bags. Others rot in the sun outside because there is little space in the refrigerators. Ridha said the bodies will be buried soon in unmarked graves if families do not come to give DNA samples.

Families claim that on their multiple trips to the morgue they have never been asked to provide such samples.

Most of the bodies from the Speicher attack remain on land occupied by Islamic State fighters, officials say.

In addition to the missing, at least 300 Nasiriyah residents have been confirmed dead since June, said Yahya al-Nassiri, the city’s governor. They are among volunteers who took up arms after a call from Iraq’s top Shiite religious cleric, Grand Ayatollah Ali Sistani.

But families of missing soldiers say they have not gotten answers to their questions about what happened to their relatives.

The governor’s home has been the target of three protests by the families. Officials say there have also been threats of attacks on Sunnis detained at the city’s al-Hout prison, which hosts some 4,000 inmates.

“Until now, the situation has been stable, but we are concerned there might be a serious reaction,” the governor said.

The threats of the family members may be nothing more than a cry for attention, but apparent reprisal killings of Sunnis are on the rise again around the country. Officials at the Baghdad morgue say they usually receive between two and seven bodies a day — a figure far smaller than during the peak of sectarian reprisals years ago, but still worrying, since many show telltale signs of execution-style killings.

An attack by suspected Shiite militiamen on worshipers at a Sunni mosque in the northern province of Diyala on Aug. 22 killed at least 60 people.

As Sayhood sat outside his home describing his loss, al-Hout prison was visible across the scrubland.

“Our sons are being eaten by these monsters, but their sons are in there in comfort, eating and sleeping,” he said, indicating the prison. “Unless we get the bodies of our sons, we will take our revenge and kill them all.”

Abigail Hauslohner in Baghdad and Mustafa Salim in Nasiriyah contributed to this report.