Workers unload Crowley shipping containers from a barge at the port in San Juan, Puerto Rico, on Sept. 26. (Alvin Baez/Reuters)

Three days after Hurricane Maria clobbered Puerto Rico, the Crowley shipping company opened its San Juan terminal and switched on its computers. When the port opened at 8 a.m. the next day, the shipping firm delivered 500 containers of commercial goods. Three of its managers had to cut their way out of their homes to get there.

Four days later, those containers and others filled with goods for stores such as Home Depot and Walgreens have been languishing at the port. Retailers hobbled by broken distribution chains and damaged stores have opened only a few outlets, and customers have had to wait in long lines.

“It’s pretty ugly out there,” said Jose Ayala, Crowley’s vice president for Puerto Rico services. “There is damage to the trucking infrastructure, to the distributors, to the supermarkets, to the roads. And then, if your infrastructure is not so damaged, and you can get a driver to the truck, there is no fuel to move the equipment.”

About 15 federal agencies, charitable groups and the Puerto Rican government are rushing to get goods shipped to the territory and to have them distributed.

Crowley is filling its ships with generators, food and water for the Federal Emergency Management Agency, and it expects to deliver more than 1,000 containers on five barges in the coming days. It dispatched more than 200 on Thursday.

President Trump, under political pressure from critics who say his administration has lagged in providing aid to Puerto Rico, flipped his position on Thursday on the 1920 Jones Act and said he would waive the requirement that vessels traveling between U.S. ports be U.S. ships. However the waiver lasts only 10 days, according to a Bloomberg News report.

While many lawmakers from both parties said the Jones Act waiver would speed assistance for Puerto Rico and reduce costs, U.S. shipping executives — including Crowley’s — and maritime unions warned that the bottleneck was on the island, not on the seas. Huge swaths of the population still lack fuel, water supplies and communication links.

John Rabin, acting administrator for FEMA Region II, said the agency has established 11 staging areas and delivered food and water to 78 municipalities. He said that 676 gasoline stations were open Thursday morning, although residents said that supplies ran out by early afternoon at many of the stations.

“Today is going to be a very difficult and hard day,” Rabin said. “Hopefully today will be a little bit better than yesterday was. And hopefully tomorrow will be a bit better.”

One of the most troublesome obstacles to relief efforts has been the electrical grid, crippled by fallen transmission and distribution lines. Though utilities belong to national groups that help coordinate out-of-state workers to help repair storm damage, so far the mainland utilities have sent crews only to help assess damage. Sue Kelly, president of the American Public Power Association, said Wednesday there was no point in sending repair crews who need food, water and shelter if they did not have the poles, wires and trucks needed.

Meanwhile, lawmakers in Congress and Puerto Rican Gov. Ricardo Rosselló have been jockeying over who should take charge of the humanitarian response effort.

Rosselló asserted that he was fully in command even as Sen. Marco Rubio (R-Fla.) and other lawmakers on the mainland stepped up demands for the appointment of a federal official to oversee relief efforts.

“Let’s make this clear, this is an operation of the government of Puerto Rico,” Rosselló said. “We set the priorities. . . . We are taking action, and there are results.”

Yet the results are mixed, even by the governor’s own assessment.

Luis Muñoz Marin Airport in San Juan has been able to handle a trickle of flights — about half a dozen a day — for several days; it was expecting nearly two dozen planes to land on Thursday. To relieve congestion, the Air Force opened airports in Ceiba and Aguadilla. Rosselló described plans to reestablish a radar in El Yunque, the national rain forest, to augment operations at all the island’s flight hubs.

Ports are slowly reopening. FEMA said it would bring in about 3.2 million meals and 2.68 million liters of water, some by air and some by sea. Only 28 percent of the island now has some cellphone reception. About 86 bank branches are open, but many people still have no cash or access to checking accounts.

FEMA said it would use barges to ship 100 fuel distribution trucks with 275,000 gallons of diesel and 75,000 gallons of gasoline. The shipment is expected to arrive Sunday.

“I wish we were in a better position but we are limited by the gravity of the situation,” Rosselló said. In Florida and Texas, where major hurricanes landed in the past two months, resources were brought in by road, but “Puerto Rico is an island. We have to bring them through boats and airplanes.”

Hence the fight over the Jones Act. Sen. John McCain (R-Ariz.), a longtime advocate of repealing the Jones Act restrictions, wrote to the Department of Homeland Security saying, “I am very concerned by the Department’s decision not to waive the Jones Act for current relief efforts in Puerto Rico, which is facing a worsening humanitarian crisis following Hurricane Maria.”

Economists agree. In 2015, Anne Krueger, former chief economist of the World Bank, wrote that the Jones Act “requires using very costly US-built ships and crews for all sea transport to and from the mainland.”

Thomas B. Crowley, chief executive of the Crowley shipping firm, said this issue should appeal to Trump, who says he wants to protect American jobs.

“If we cut some American jobs, replace them with foreign labor and save a few pennies on the delivered goods, then perhaps you could get the answer swayed to yes, but no one has ever made a factual case that this is true,” Crowley said in an interview.

After the president announced the waiver, Crowley said in an email: “We understand the waiver will be temporary. In the meantime, we hope people will take the time to learn what our American vessel crews, dock workers and truck drivers are doing 24/7 to bring help to Puerto Rico. Americans responding to Americans in need.”