Panama City, Panama (CNN) -- Four days have passed since Panamanian authorities discovered undeclared military weapons hidden aboard a North Korean ship, and the painstaking process of examining the entire vessel is crawling at a snail's pace.
The ship has five cargo holds, only one of which has been emptied as of Thursday.
"The technicians on board have told us that this cargo was loaded in a way that makes it difficult to unload," Panamanian Security Minister Jose Raul Mulino said.
The North Korean crew had resisted the Panamanian authorities and cut the cables to the onboard cranes. Panamanian investigators brought their own cranes, but removing the containers inside the cargo holds has been an "odyssey," Mulino said.
The ship originated in Cuba, and the Cubans have admitted to owning the military equipment, claiming it was being sent to North Korea to be repaired and returned.
But many questions remain. If the weapons were not a secret, why were they hidden under sacks of sugar? Why the did the captain attempt to commit suicide?
A public prosecutor is charging the captain and 35 North Korean crew members with illegal possession of weapons and international arms trafficking, Panamanian government spokesman Eduardo Camacho said.
North Korean officials, meanwhile, asked for Panama to release the cargo ship and let the crew go.
Panama has formally asked the United Nations for guidance on how to handle the case.
"For us, it is important to finish this operation, wait for the United Nations to come, and they will decide" how to proceed, Mulino said. "Panama is completely transparent in this; we have no experience in dealing with this type of problem."
Because it is pursuing nuclear weapons, North Korea is banned by the United Nations from importing and exporting most weapons.
Scene at the port
At the port of Manzanillo, inspectors opened shipping containers in front of reporters. Heavily armed troops stand guard.
In the first cargo hold, six shipping containers were found underneath sacks of brown sugar, in two stacks of three. The tops of some of the containers were caved in because of the weight of the sugar.
Inside the containers lies the military equipment.
Inspectors walked inside the containers, taking pictures.
Cuban officials have described the materiel as "240 metric tons of obsolete defensive weapons" sent to North Korea "to be repaired and returned to Cuba."
The equipment was manufactured in the mid-20th century and included two anti-aircraft missile systems, nine missiles in parts and spares, two MiG-21 jets and 15 motors for this type of airplane, the Cuban foreign ministry said.
U.S. involved in investigation
The United States and Panama had been tracking the ship as it crossed the Panama Canal to Cuba and then back, two U.S. officials said.
And a U.S. State Department spokeswoman said Wednesday that the United States would help in the investigation.
The Panamanians asked the United States for imaging equipment and technicians to fully examine the boat and determine what is on board, according to a U.S. official who declined to be identified because the person was not authorized to speak publicly.
Speculation has surged since Panama announced its find, with some warning that it was a troubling sign of weapons deals between North Korea and Cuba, and others disputing whether any dangers lay within the antiquated haul.
Cuba says the weapons are "obsolete." And experts who identified early Cold War relics such as the Soviet-designed SA-2 air defense system among the ship's cargo say that's not far from the truth.
"Today there is no reason for any Western pilot to be hit by an SA-2. If you get caught by one of them, you've done something bloody stupid, or you've got very bad luck," said James O'Halloran, editor of Jane's Land Based Air Defence and Jane's Strategic Weapon Systems. "No modern country wants to be seen with those."
But others saw the weapons haul as a more ominous sign.
In a letter to U.S. Secretary of State John Kerry, Sen. Marco Rubio of Florida, a frequent critic of the Cuban government, described the weapons shipment as a "flagrant violation of multiple United Nations Security Council Resolutions."
CNN's Catherine E. Shoichet in Atlanta and journalist May Lee in Panama contributed to this report.