In the wake of Hurricane Maria, Puerto Rico’s 3.4 million American citizens remain almost entirely without electrical power — but it’s not because their power plants are blown. The problem is that roughly 80 percent of transmission lines, which take power from the plants to distribution centers, are down. Nearly all the local power lines that run to residences and businesses have likely also been destroyed.

The damage is so severe that simply repairing the electrical grid may not be an option. “We really should think in terms of rebuilding at this point,” says Ken Buell, director of Emergency Response and Recovery with the US Department of Energy. Paying for it will be a challenge, however: the Puerto Rico Electric Power Authority, or PREPA, is bankrupt, with at least $9 billion in debt, The New York Times reported in July. “They’re saying as far as economic impact, we're talking probably billions of dollars of impact,” Buell says. “So it's a big deal.”

Right now, power in Puerto Rico is coming from generators, and it’s not clear how long the island will be relying on them. PREPA has conducted “a good portion of their damage assessment, which is the first step in power restoration,” Buell says.

Hurricane Maria, a Category 4 storm, tore across the island on September 20th, destroying homes, ripping down power lines, and killing at least 10 people so far. It didn’t severely damage the power plants, though, Buell said after speaking with a PREPA representative Monday morning. “One of the plants is on the top eastern edge of the island and actually took what appeared to be a direct hit from Maria,” he says. “So the fact that these plants are okay to start is a good thing.”

The big problem is the catastrophic damage to most of the power lines that carry electricity from the power plants to the power distribution centers in major population centers. These run over mountains, and only two or three underground lines weathered the storm, Buell says. The local power authority hasn’t finished assessing the damage to power lines that run from the distribution centers to residences and businesses, “but they're assuming that it's near 100 percent,” Buell says.

Since Puerto Rico is an island, damage to its ports and airports are hindering efforts to send help. The ports are reopening with a limited capacity, but during the daytime only, Buell says. “There’s a lot of stuff on the water and it's very difficult to navigate.”

The airport is running on a generator and is open for daytime operations at the moment, but the radar systems are still down, Buell says. Planes cannot leave the mainland until they have a guaranteed spot on the tarmac at the San Juan Airport, which at one point created an 11-hour delay to land and unload supplies. “So there’s this huge backlog of getting stuff onto the island that you wouldn’t see typically with a storm that’s on the mainland.”

Buell says that PREPA confirmed to him on Monday morning that its first priority is to reconnect hospitals and government facilities to the electrical grid. “Right now, they’re operating on generators — and that creates a potential fuel problem,” Buell says. At some hospitals, back-up generators have failed; others are running short on diesel, Reuters reports. To combat looting, armed guards are delivering precious fuel supplies, the director of the heart transplant program at Centro Cardiovascular in San Juan told Reuters.

“The government’s main focus is on the health and safety of the citizens,” Buell says. “Security is an issue. People are getting pretty desperate.” He doesn’t know how long it will take to restore power to the island, with roads left impassable and communication still so limited. The mayor of San Juan has said she expected it could take four to six months for the lights to turn back on — but PREPA’s chief executive told the New York Times he expected three to four months, at most.

That’s a lot of time to be without power — especially as Puerto Rico attempts to recover from the storm. “There's no money coming in and you've got these big bills ahead,” Buell says. “It's going to be a problem.”

Correction: An earlier version of this post used the wrong personal pronoun for the mayor of San Juan. The mayor of San Juan is a woman, Carmen Yulín Cruz. We regret the error.