Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

Terror experts and U.S. officials say a Boko Haram assault this week on a small city on the northern border of Nigeria may have killed as many as 2,000 civilians.

Many survivors fled into the nearby waters of Lake Chad, where some drowned and where others remain marooned on small islands, menaced by hippos, said a local government official.

District leader Musa Alhaji Bukar told the BBC 2,000 residents from Baga and 16 other towns had been killed by the radical Islamist terror group, and that Baga was now “virtually nonexistent,” which would make the massacre among the most deadly terror attacks in history.

In an interview with NBC News on Thursday, Ahmed Zanna, a local senator, said the militants razed Baga and the other communities. "These towns are just gone, burned down," he told NBC News via telephone. "The whole area is covered in bodies."

Zanna said that more than 2,000 people were unaccounted for, and residents who fled the towns reported the killing had been going on since Boko Haram overran a nearby military base Saturday.

American experts say such reports are credible and that officials from Nigeria’s central government, who have put the body count in the hundreds, are prone to underestimating death tolls.

A U.S. counterterrorism official said the tally of 2,000 deaths "may not be that far off."

John Campbell, a former ambassador to Nigeria who is now a senior fellow at the Council on Foreign Affairs, also agreed that the estimate of 2,000 was “not unreasonable,” given the “magnitude” of the attack.

“We have to go by previous pattern and experience,” said Campbell. J. Peter Pham, director of the Atlantic Council Africa Center, also told NBC News he believed the number was “credible.”

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Follow NBC News Investigations on Twitter and Facebook.

All three, however, cautioned that getting an accurate death total was nearly impossible.

The attacks, a coordinated assault on the city and nearby towns, were brutal even for a terror group with a history of mass killings. According to the U.S. counterterrorism official, Boko Haram militants went door-to-door killing families, then strategically placed improvised explosive devices in the streets to funnel survivors into areas where firing squads were pre-positioned. "They were mowed down" by automatic weapons fire, the official said.

The villages were then set on fire, and militants moved on to other areas to repeat the process, officials told the BBC.

Those who survived the deadly gauntlet tried to take refuge in Lake Chad, huddling on islands near the shallow lake’s marshy shore.

"These are not stable islands, more like sand bars," explained Pham. "The topography, with the marshes and hippos, gives you a flavor of the misery those who've escaped are facing."

Wednesday's attack on Baga came four days after Boko attacked a Nigerian military base near Baga. Ironically, Baga was supposed to be the main base for joint operations against the terrorist group, the centerpiece of a French-sponsored alliance of Nigeria, Cameroon, Niger and Chad. It was overrun in a day. Nigeria troops, the only ones on hand, once again abandoned their post -- leaving the city and villages open to attack.

Experts said Boko Haram, like ISIS in Syria and Iraq, is trying to amass territory. Boko Haram has already declared a caliphate 100 miles south of Baga in Gwoza, and may be trying to expand its geographic reach beyond Nigeria's borders.

"They're clearing out people, getting them flexibility and a broader space to operate in," said the U.S. counterterrorism official.

Pham said that Boko Haram has treated civilians in the far north more cruelly than those near Gwoza. The violence in the north, said Pham, is meant “to create havoc to meet their military goals."

Campbell said that Boko Haram now controls all the border crossings in the area between Nigeria and its neighbors, Cameroon, Chad and Niger. Only last week, Campbell noted, Boko Haram leader Abubakr Shekhau released a video in which he called for attacks in Cameroon, which like Nigeria is run by a Christian government but has a substantial Muslim population.

In addition, the officials and experts agreed the attacks give Boko Haram a strategic advantage should they want to gain control of Maiduguri, a city of one million in the northeast corner of Nigeria, and the city where Boko Haram was born. Campbell said that he doesn't believe that Boko Haram could overrun Maiduguri, but suggests they could control it -- with sleeper cells already inside the city, some disguised as refugees from nearby fighting.

"When you suggest that Maiduguri is the target, understand this: Boko Haram is already in Maiduguri, with reports of black flags flying over abandoned government buildings," said Campbell, referring to the black and white radical Islamist banner.

London-based human rights group Amnesty International said the massacre could be the “deadliest act” ever perpetrated by Boko Haram.

"If reports that the town was largely razed to the ground … are true, this marks a disturbing and bloody escalation of Boko Haram's ongoing onslaught against the civilian population," said Amnesty’s Nigeria researcher Daniel Eyre in an emailed statement. "The attack on Baga and surrounding towns looks as if it could be Boko Haram's deadliest act in a catalogue of increasingly heinous attacks carried out by the group."