Big Jake

In 1909, there is a raid on the McCandles family ranch by a gang of ruthless outlaws led by John Fain. They massacre the ranch hands and kidnap Little Jake, the grandson, leaving a ransom note and heading back for Mexico, where they have been hiding out. Martha, the head of the family, is offered the help of both the army and the Texas Rangers in hunting the gang. She replies that this will be "a harsh and unpleasant kind of business and will require an extremely harsh and unpleasant kind of person to see it through." In consequence, she sends for her estranged husband, the aging Jacob "Big Jake" McCandles, a near-legendary gunfighter who wanders the west with his Rough Collie, simply named Dog.
When Jake arrives by train, he and Martha discuss a plan to take the ransom to the kidnappers, a million dollars in a big red strongbox, although Jake warns that, "Pay or not, we run the risk of never seeing the boy again". Then his son Michael rides up on a motorcycle, bringing word that he has seen the kidnappers in the Chilicothe Canyon. The Texas Ranger captain is present and offers the services of his men, equipped with three REO touring cars. They, however, are ambushed and their cars put out of action. Jake, preferring the old ways, has followed on horseback, accompanied by an old Apache associate, Sam Sharpnose. He is now joined by his sons, Michael and James, with whom his relations are tense because of his desertion of the family ten years before.
That night, Fain rides into their camp to make arrangements for the handover, telling Jake that they will âsend the boy's body back in a basket" if anything should go wrong. Both men deny any personal stake in the business, each claiming to be "just a messenger boy". The family party crosses into Mexico the next day and checks into a hotel. Knowing that they have been followed by another gang intent on stealing the strongbox, Jake sets a trap for them and they are all killed. During the attack, the chest is blasted open, revealing clipped bundles of newspaper instead of money. Michael and James become suspicious of Jake and the three slug it out, but Jake assures them that it was both his and Martha's idea. James fears for Little Jake's life, but Jake tells them they'll have to go in anyway.
A thunderstorm breaks and Pop Dawson, one of the outlaws, arrives to give them the details of the exchange. He warns them that a sniper will have a gun trained on the boy in case of a double-cross. Jake arranges for Michael to follow after them to take care of the sharp shooter and convinces Dawson that he had been killed in the fight. At the hideout, Jake is led in alone to where Fain and four other gang members are waiting, one of them holding a shotgun on the boy. Jake tosses the key of the chest to Fain, who opens it up to discover that he has been tricked.
Fain orders his brother Will to kill the boy but he is shot by Jake. Dog is wounded by the sniper and Jake is wounded in the leg before Michael kills him. Jake tells the boy to escape but Little Jake is hunted by the machete wielding John Goodfellow, who has already hacked Sam to death. Dog comes to the rescue and is himself killed before Big Jake arrives and impales Goodfellow on a pitchfork. Fain rides up and is preparing to finish off the two of them when Michael arrives from where he had been waiting in ambush and blasts him off his horse. Before he dies, Fain asks, "Who are you?" When Jake answers, "Jacob McCandles," Fain says, "I thought you was dead," as have other characters during the course of the film. âNot hardly," Jake replies.
With Little Jake rescued, and the broken family bonded, they prepare to head home.

Question: How was the strongbox opened?
Answer: It was blasted during an attack.