Just offshore from the towering national monuments of Washington, a tall crane now hovers over the Potomac River.

It casts a large shadow over the ongoing effort to understand what caused the collision of a passenger jet and a military helicopter last Wednesday as a new phase of the recovery started on Monday (Tuesday AEST).

It’s the deadliest US aviation disaster in more than 20 years.

A jet engine was the first piece of debris to emerge from the frigid water on Monday, slowly raised by the crane.

Video taken from shore showed salvage workers guiding it gently to the deck of a barge.

From a distance, most of the engine’s cowling and exhaust nozzle appeared intact.

Here’s the latest on the recovery operations and the investigation into the cause:

Removing wreckage to take over a week

The salvage work began after divers spent the weekend getting a view of what people on the surface cannot see – the full extent of the underwater debris field – developing a plan to recover the jet wreckage that they believe will take the rest of the week.

Pulling out the wreck of the helicopter is scheduled to take another four days.

But the US Army Corps of Engineers – which is managing the recovery operation – says the exact timeline cannot be certain because not all victims have been identified.

If more human remains are found during the wreckage removal, they will pause.

“The dignified recovery of missing flight passengers and personnel takes precedence,” Col. Francis Pera, commander of the Corps’ Baltimore District, said Sunday.

The remains of 55 of the 67 victims have been identified, according to a news briefing Sunday.

Those victims included 28 athletes, coaches and family members associated with US Figure Skating.

Many of them were participants in the national championships in Wichita and a development camp which followed, the organisation said.

Recovery crews began their work under clear skies but will be working in water that will hover around just 36 degrees.

By Wednesday, inclement weather is forecast to return to Washington, with a possibility of rain and sleet.

New black box data to be released

The first sign that the pilot of one of the aircraft may have seen the other before the collision came to light over the weekend: The National Transportation Safety Board says the jet’s flight data recorder has an indication that the plane’s nose came up just before the disaster.

“At one point very close to the impact, there was a slight change in pitch, an increase in pitch,” NTSB member Todd Inman said at a Saturday evening news conference.

A key question in the investigation is whether the Black Hawk helicopter was higher than the 200-foot altitude limit it was supposed to observe as it flew a designed route along the eastern bank of the Potomac.

Inman said the Bombardier jet’s flight data recorder gives an altitude reading of about 325 feet at the time of impact, but air traffic controllers never had an indication that the helicopter went above 200 feet.

The NTSB recovered the black box of the Black Hawk helicopter, the agency said on Sunday.

Board chairperson Jennifer Homendy told Fox News on Monday information was recovered from the flight data recorder.

“We do have that data,” Homendy said.

“We will be able to provide that in our press briefing later today.”

The black box is critical, Homendy said, because the DC tower radar does not provide a constant readout of aircraft altitude, only updating every five seconds.

“What we’ve done is pulled additional data to get better, more granular information to understand the altitude,” she said.

“The issue is going to become did the readout come from the helicopter? Was it tower equipment that misinterpreted?” said CNN transportation analyst Mary Schiavo, former inspector general at the Department of Transportation.

If the radar showed bad readings, Schiavo said investigators will want to look at maintenance records.

“The air traffic controllers don’t repair that tower equipment,” Schiavo told CNN’s John Berman.

“That is handled by contractors, and so now we’re looking at another group of people. Who was repairing that tower equipment?”

A preliminary report from the NTSB is still likely to be a month away.

Black Hawk and tower actions under review

The altitude of the helicopter is still a key question to be answered, Transportation Secretary Sean Duffy told CNN’s “State of the Union” on Sunday, but there are others, as well.

“Were the pilots of the Black Hawk wearing night vision goggles? Did it affect their peripheral vision or perception?” he said.

“What was happening inside the (air traffic control) tower?” Duffy added.

“Were they understaffed?”

On the night of the crash, a single controller was handling approach traffic for both planes and helicopters, an air traffic control source told CNN.

Normally those jobs are performed by two different people, although the source and the air traffic controllers’ union said it was not uncommon for those duties to be combined.

Even as evidence continues to be collected from tapes and pulled from the water, the former director of the FAA’s Office of Accident Investigation says he’s confident that the cause of the disaster will be unambiguous.

“We have radar data, we have eyewitnesses, and we have all the wreckage,” Steven Wallace told CNN’s Phil Mattingly. “There’s nothing missing.”