BERLIN — A day after a bus plunged off a road on the Portuguese island of Madeira, killing 29 tourists, possibly all Germans, Chancellor Angela Merkel of Germany said she was “saddened and distressed” by the accident, as her nation awaited word of the victims’ identities.

The Portuguese Foreign Ministry said on Thursday that all those killed had been German citizens, but Berlin said it could not yet confirm that information. The German government took the unusual step of readying a medical evacuation plane, however, and will send the foreign minister, Heiko Maas, to help determine how many of the victims were German.

The German ambassador to Portugal, Christof Weil, also traveled to Madeira on Thursday morning, ahead of Mr. Maas, to assist with the identifications. Portuguese officials said the identification process would not be completed until Saturday.

As Germany awaited answers about the cause of the accident — the bus drove off the road and landed on an empty house, leaving 27 others injured — the authorities in Portugal declared three days of national mourning, and German politicians expressed their condolences.

“It is shocking that the Easter holiday had become a tragedy for so many people,” Mr. Maas said in a statement. “They were suddenly killed or struggled with serious injuries,” he added.

Ms. Merkel sent her condolences while she was away for the funeral service of her mother, who died on April 6.

The tourist bus was carrying at least 56 people on Wednesday night when it went off the road and tumbled down a steep incline on the island, Portuguese officials said. Twenty-eight people died almost instantly, and one person died at a hospital hours later, physicians at Dr. Nélio Mendonça Hospital said at a news conference.

The doctors said the injured had been 40 to 60 years old. They were taken to a medical center in Funchal, the island’s capital. Nine of the injured were released on Thursday morning. Several were still in intensive care a day after the accident, the doctors said.

Two Portuguese citizens onboard — presumably the driver and the guide — survived the accident and were in stable condition, according to hospital officials.

An unidentified woman who was on the bus said on Thursday in a televised interview in Portugal that during the accident, passengers went into a “fetal position” and “that’s why we survived.”

Portuguese officials hauled away the wreckage of the tour bus from the crash site on Thursday.

At a news conference, Helge Braun, a minister in Ms. Merkel’s government, asked the public and the news media to respect the relatives of the victims, who needed to be informed before the victims could be named publicly.

Most of the passengers were German citizens who had booked their Easter vacation with the same tour company, according to reports in the German news media. The bus was said to have been taking the group to a restaurant when it drove off the road several hundred yards into its route.

Security video footage shows the white bus tumbling down the hill. It came to rest after falling on the house, according to officials. The accident took place on a treacherous road that is difficult for larger vehicles to navigate, officials said.

Maria de Lurdes Correia, an official in Madeira, said a judicial investigation had been opened into the cause of the crash. The Portuguese authorities said the bus was about six years old and had been handled by an experienced, 55-year-old driver.

Madeira, about 400 miles off the African coast, is a popular holiday destination for European tourists. In 2016, the island hosted about 1.2 million international visitors — more than four times the island’s population.

In 2015, more than four million German vacationers had bus trips abroad lasting longer than five days, according to the International Coach Tourism Federation. Among tourists who fly to a destination, 23 percent use tour buses to visit the local sights, the federation said.

“We Germans are rather social; they like traveling together in the bus,” said Dieter Gauf, the former managing director of the federation.