The U.S. Coast Guard ended its search for survivors of the El Faro container ship at sunset Wednesday, shifting the focus to the hunt for the wreckage and clues to what caused the deadliest U.S. shipping disaster in more than three decades.

Families were informed of the decision about 1 p.m. Wednesday, ending six days of intense waiting since the Coast Guard learned last Thursday that the vessel was in distress.

“The decision to end a search is painful, and is based on the art and science of search and rescue,” Lt. Cmdr. Gabe Somma said during a news conference in Jacksonville, Florida.

The 33 crew members, including four Mainers, lost in the El Faro sinking eclipses the 31 people who died in 1983 when a bulk carrier sank off the coast of Virginia, prompting major changes in shipping safety standards and water-rescue techniques.

President Obama issued a statement Wednesday evening offering his condolences to the families of the crew lost on the El Faro, and emphasizing the importance of what they and other mariners do for Americans’ economic prosperity.

“The investigation now underway will have the full support of the U.S. government, because the grieving families of the El Faro deserve answers and because we have to do everything in our power to ensure the safety of our people, including those who work at sea,” the president said.

FOCUS ON SHIP’S DATA RECORDER

Federal investigators searching for clues in the El Faro disaster are expected to focus on the potentially difficult and costly task of retrieving the ship’s voyage data recorder from 15,000 feet below the surface.

The National Transportation Safety Board is leading the inquiry, and will coordinate with the U.S. Navy to find and retrieve information from the ship. The ship’s location is unknown, but it is believed to be resting more than 30 miles off the northern coast of Crooked Island in the Bahamas.

“Our investigation is well underway,” NTSB Vice Chair Bella Dinh-Zarr said during a news conference Wednesday afternoon in Jacksonville. “We will be here as long as it takes.”

The Navy’s Office of the Director of Ocean Engineering, Supervisor of Salvage and Diving will conduct the search using sonar-based equipment and listening devices to detect the emergency ping sent out by the data recorder, Dinh-Zarr said.

Anthony Chiarello, president and CEO of ship owner TOTE Inc., made a brief statement but didn’t take any questions Wednesday. Chiarello sent his prayers to the families of the crew, and vowed to cooperate fully with the NTSB investigation.

“I will tell you as a TOTE family, we too are grieving,” he said. “There will be many legacy learnings from the NTSB investigation.”

The Coast Guard released a list of El Faro’s crew, which was provided by TOTE Maritime Puerto Rico, the subsidiary that operated the 790-foot ship. The four Mainers were Michael Davidson, 53, of Windham, the ship’s captain; Danielle Randolph, 34, of Rockland; Michael Holland, 25, of Wilton, and Dylan Meklin, 23, of Rockland, who graduated in May. The crew also included a member of Maine Maritime Academy’s class of 2011, Mitchell Kuflik of Brooklyn, N.Y. Most of the other crew members were from Florida, and there also were five Polish nationals aboard.

Investigators are focusing on two debris fields, one of which is believed to be close to where the ship sank.

The data recorder is designed to send out auditory pings for roughly 30 days after it hits the water.

If found, the so-called black box likely would have to be recovered by the Navy, which has robotic retrieval equipment capable of operating as far as 20,000 feet below the surface.

The model of the data recorder on the El Faro is not the most sophisticated in use, but probably would provide the ship’s last known position, speed and heading, as well as audio recordings from the bridge and from radio traffic.

At nearly 3 miles below the surface, the wreckage is at the outer limits of a typically feasible search operation. But because of the significant loss of life, it is expected that authorities will make every effort to locate the ship and find the data recorder that could explain how and why it went down, said David DeVilbiss, vice president of marine casualty response for Global Diving and Salvage Inc.

Locating the wreck will probably be the easy part, DeVilbiss said. Unlike a crashed airliner, which breaks up on impact, the wreck of the El Faro likely will be found largely intact.

Sonar equipment will be used to find the wreckage, and an acoustical ping locater will help identify the signal from the data recorder.

The operation will not be cheap, DeVilbiss said, and could require a full cadre of equipment at the Navy’s disposal.

“It all comes down to money, at the end of the day,” he said. “It’s very unusual to be mucking around with ships in 14,000 or 15,000 feet of water.”

The four Mainers aboard the El Faro all graduated from Maine Maritime Academy, whose students and faculty have been following news of the ship’s sinking with heavy hearts.

In a brief statement, MMA President William Brennan said the school community “will grieve this together,” and the academy will continue to offer counseling services to students.

“I have no doubt this will prove to be a learning experience for all of our students and all of our faculty and staff,” Brennan said. “We train for this.”

ANSWERS MAY TAKE 18 MONTHS

Lessons from the disaster are not likely to come any time soon, however.

The NTSB’s investigation could take up to 18 months, although critical facts learned before the investigation report is finalized could be released earlier at the NTSB’s discretion.

The El Faro left Jacksonville on Sept. 29 bound for San Juan, Puerto Rico, a journey of about 1,300 miles to the southeast. At the time, the storm gathering in the Atlantic Ocean was still classified as a tropical storm. On board the ship were 391 containers topside and 294 trailers and vehicles below deck.

The National Weather Service issued an advisory upgrading then-Tropical Storm Joaquin to hurricane status while the ship was several hundred miles into its voyage. The vessel remained on its course through seven additional hurricane advisories over the next 21 hours. On Oct. 1, however, the El Faro lost propulsion and was unable to evade Joaquin, which grew to a Category 4 storm with winds that topped 130 mph, leaving the ship helpless to move from its location as Joaquin bore down.

Participating in the investigation are the Coast Guard, American Bureau of Shipping, TOTE Inc. and officials from Poland, whose five citizens were aboard to do work related to a pending retrofit of the ship’s engine room, said U.S. Rep. Chellie Pingree.

TOTE is cooperating with the investigation, Pingree said, and so far has provided the NTSB access to the El Faro’s sister ship, the El Yunque, which could help investigators learn about the characteristics of how the ship behaves during rough weather.