[2] Over the next 24 hours, Maria explosively strengthened to Category 5 status just 15 miles (24 km) east-southeast of Dominica,[3][4] before making landfall on that island early on September 19.

[5] Despite some slight initial weakening, Maria reached peak intensity early on September 20 while roughly 30 miles (48 km) south of St. Croix, with maximum sustained winds of 175 mph (282 km/h) and a minimum central pressure of 908 mbar (26.8 inHg).

[8] Maria made landfall in Puerto Rico, just south of Yabucoa, around 10:15 UTC, with sustained winds of 155 mph (249 km/h) and a central pressure of 920 mbar (27 inHg).

[26] Extensive damage occurred to hundreds of thousands of buildings throughout Puerto Rico due to the high winds, heavy rainfall, storm surge, wave action and landslides.

[27] Storm surge inundation as high as 9 ft (2.7 m) above ground level occurred along the southeastern coast, and flash flooding stemming from floodgate releases at La Plata Lake Dam converged on the town of Toa Baja, trapping thousands of residents.

[36] The remaining structures on the island of Culebra were extremely vulnerable to Maria's powerful winds after having recently experienced major damage due to Hurricane Irma, causing the complete destruction of many wooden houses, along with blown off roofs and sunken boats.

[48] New and expecting mothers experienced a greater exposure risk to health issues because Hurricane Maria created an environment for pathogens and environmental toxins to thrive.

[58][59] In the months following Maria, media outlets, politicians, and investigative journalists questioned the official death toll of 64 from the Government of Puerto Rico.

By the end of November, the Puerto Rican government maintained that their report of 55 fatalities was the most accurate despite ample contrary evidence collected by media and investigative journalists.

[64] On August 28, 2018, the Government of Puerto Rico revised the official death toll to be 2,975 people, ranking Maria as one of the deadliest hurricanes in United States history.

[74] The Guajataca Dam was structurally damaged, and on September 22, the National Weather Service issued a flash flood emergency for parts of the area in response.

[66] The Federal Emergency Management Agency planned to open an air bridge with three to four aircraft carrying essential supplies to the island daily starting on September 22.

[29] Beyond flights involving the relief effort, limited commercial traffic resumed at Luis Muñoz Marín International Airport on September 22 under primitive conditions.

[81] The United States Transportation Command moved additional personnel and eight U.S. Army UH-60 Black Hawk helicopters from Fort Campbell, Kentucky to Luis Muñoz Marín International Airport to increase distribution capacity.

[88] National Guard troops were activated and deployed to Puerto Rico from Connecticut, Georgia, Iowa, Illinois, Kentucky, Missouri, New York, Rhode Island, and Wisconsin.

[89] On September 29, the hospital ship USNS Comfort left port at Norfolk, Virginia to help victims of Hurricanes Irma and Maria, and arrived in San Juan on October 3.

"[92] After remaining offshore for three weeks, the Comfort docked in San Juan on October 27, briefly departing only once to restock at sea from a naval resupply ship.

[93] On September 27, the Pentagon reopened two major airfields on Puerto Rico and started sending aircraft, specialized units, and a hospital ship to assist in the relief effort; Brigadier General Richard C. Kim, the deputy commanding general of United States Army North, was responsible for coordinating operations between the military, FEMA and other government agencies, and the private sector.

[96] On September 29, the amphibious assault ship USS Wasp which had been providing relief activities to the island of Dominica was diverted to Puerto Rico.

[97] As of September 30, FEMA official Alejandro de la Campa stated that 5% of electricity, 33% of the telecommunications infrastructure, and 50% of water services had been restored to the island.

[102] With centralized fossil-fuel-based power plants and grid infrastructure expected to be out of commission for weeks to months, some renewable energy projects were in the works, including the shipment of hundreds of Tesla Powerwall battery systems to be integrated with solar PV systems[103] and Sonnen solar microgrid projects at 15 emergency community centers; the first were expected to be completed in October.

Prominently, Jennifer Aniston pledged a million U.S. dollars, dividing the amount equally between the Red Cross and The Ricky Martin Foundation for Puerto Rico.

[110] On October 13, the Trump administration requested $4.9 billion to fund a loan program that Puerto Rico could use to address basic functions and infrastructure needs.

[121] On February 11, an explosion and fire damaged a power substation in Monacillo,[122] causing a large blackout in northern parts of the island including San Juan, Trujillo Alto, Guaynabo, Carolina, Caguas, and Juncos.

[132] San Juan Mayor Carmen Yulín Cruz called the disaster a "terrifying humanitarian crisis" and on September 26 pleaded for relief efforts to be sped up.

[134] General Joseph L. Lengyel, Chief of the National Guard Bureau, defended the Trump Administration's response, and reiterated that relief efforts were hampered by Puerto Rico being an island rather than on the mainland.

"Oxfam rarely responds to humanitarian emergencies in the US and other wealthy countries, but as the situation in Puerto Rico worsens and the federal government's response continues to falter, we have decided to step in.

"[109] In response to a request for clarification on the tweet from Governor Rosselló, John F. Kelly assured that no resources were being pulled and replied: "Our country will stand with those American citizens in Puerto Rico until the job is done".

[11] After visiting Puerto Rico about two months after the hurricane, Refugees International issued a report that severely criticized the slow response of the federal authorities, noted poor coordination and logistics, and indicated the island was still in an emergency mode and in need of more help.

DHS Inspector General John Roth led the FEMA audit while Governor Rosselló called for a second review by Puerto Rico's Office of Management and Budget.