Editor's note: Just before the United States entered each of the world wars, New London was visited by a foreign submarine. Neither was what it seemed, and both brought wartime intrigue to the Whaling City. November marks the 100th anniversary of one visit and the 75th of the other. This story is drawn mostly from the research of Frederick H. Hallett and the archives of The Day. Second of two parts.

The huge submarine docked at State Pier was from France, but in 1941 there were two ways to be French, and that was the central fact in the life of this strange vessel.

One way was to remain loyal to the country's nominal government, which had capitulated to the Nazis and collaborated from its seat in Vichy. The other was to serve Free France, the government-in-exile that fought with the Allies against Germany.

The Surcouf, at 361 feet the largest submarine in the world, flew the flag of Free France when it arrived in New London in November 1941. But things were a little more complicated than that.

To the public, which read The Day's minimal coverage, things seemed straightforward. Though the sub's mission was not disclosed, to all appearances it was a goodwill visit.

Officers were entertained by a French Catholic group at a home on Post Hill Place. They also spoke at a reunion of the Bulkeley School Class of 1923. In an unscripted moment, five sailors crashed their car on Winthrop Street after a night of drinking.

The apparent crowning event of the visit was the wedding of a crewman and his fiancée at the Crocker House.

But Surcouf wasn't in New London for a wedding. The submarine had just undergone months of overhaul at the Portsmouth Naval Shipyard and was here to quietly test the repairs.

The trials didn't go well, and after Surcouf collided with another vessel, they were cut short. Surcouf departed, and that was that.

Nothing about the submarine's two weeks here, even the part the public didn't know about, seemed extraordinary. But that began to change, after the fact, three months later.

In February 1942, Surcouf left a British base in Bermuda, bound for the Panama Canal. It was never seen again.

* * *

Launched in 1929 and named for a pirate, Surcouf was to be the lead ship in a class of commerce-raiding submarines. It was built with exotic features like an on-board airplane and hangar, and a prison for 40 captives.

But with basic design flaws, the sub was slow to dive and unstable in rough seas. The rest of the class never materialized, and the one-of-a-kind Surcouf was soon outpaced by changes in submarine technology and strategic thinking.

When France fell to the Nazis in 1940, the British turned on their former ally and attacked its navy to keep it out of German hands. In addition, French ships in British ports were seized.

All surrendered peacefully except one. With Surcouf docked in Plymouth, its crew fired on a British boarding party, and four men were killed.

The crew then faced the choice of being repatriated to occupied France or fighting the Nazis from exile. The decision was not clear-cut, and some left while others stayed.

The ship was then turned over to Free France and its crew filled out by other exiled sailors, many of whom had no submarine experience.

Thus Surcouf entered World War II as an unreliable relic with a crew that was untrained and unsure of its allegiance. Useless as a warship, it was nonetheless prized as a symbol of bygone French military might.

The British and Free French struggled to find something for Surcouf to do, and the ship was plagued by technical problems, poor morale and incompetence.

"I ... felt the boat was unsafe and should not be allowed to go to sea again because the captain and crew were incapable of operating it satisfactorily," wrote a British liaison officer aboard.

Surcouf spent three months under repair in Portsmouth, followed by the trials in New London. Afterward it helped liberate two French islands off Newfoundland from Vichy, which caused diplomatic problems for the allies.

When Surcouf was assigned to the South Pacific and disappeared en route, the British were probably relieved.

* * *

The story usually went like this: While in New London posing as Free French, Surcouf would sail into Long Island Sound to secretly supply and refuel German U-boats.

The trips supposedly aroused suspicion, prompting the U.S. Navy to send a sub to see what Surcouf was up to. Caught in the act, both Surcouf and a U-boat were attacked and sunk.

Conspiracy theories were bound to crop up after Surcouf's disappearance, given the sub's uncertain loyalty and checkered career.

For some reason, the most popular story centered on New London. This is hard to understand given that little happened while Surcouf was here. Even stranger, the sub's whereabouts were well-documented in its last three months after it left State Pier.

But the sea trials provided fodder for speculation, and the story was still being repeated a half century later.

In one version, which a Mystic man recalled in The Day in 1997, the sinking was prompted by a U.S. attack on a U-boat off the coast. A bread wrapper that floated to the surface was marked as having come from a U.S. bakery three days earlier. Since Surcouf had just left New London, the wrapper was taken as a smoking gun.

The U.S. sub that supposedly sank Surcouf is usually identified as either Mackerel or Marlin, experimental vessels built at Electric Boat and based in Groton for most of the war. The subs were used in the filming of a movie called "Crash Dive."

In 1989 a retired Navy man who said he was there recalled that one day filming was cut short as Mackerel and Marlin abruptly put to sea. When they returned, rumors flew that they had dispatched Surcouf.

It's a good story, but filming of "Crash Dive" in Groton didn't start till the summer of 1942, months after Surcouf vanished.

In 1965 the tale got new life when a diver named Lee Prettyman Jr. announced he had found the wrecked sub in Long Island Sound.

His story was that a mysterious U.S. Naval officer with a German accent claimed to be a survivor of the U-boat sunk along with Surcouf. Over drinks, he supposedly told Prettyman where to find the French sub.

"It was a weird, eerie experience, swimming alongside it — almost like robbing a grave," Prettyman said.

He declined to reveal the location till the time was right. Newspapers were still writing about the claim in the 1970s, but somehow, the right time never arrived.

* * *

Eighty miles north of the Panama Canal, the American freighter Thompson Lykes sailed through the night under blackout conditions.

When the helmsman spotted something in the water ahead, the captain ordered right full rudder. Unable to avoid a collision, the crew felt an explosion, saw flames and heard cries for help.

The Lykes, intact, radioed for assistance and was joined by two Navy ships, but their searches found no wreckage or bodies, and what the freighter hit remains a mystery.

The collision occurred Feb. 18, 1942, days after Surcouf left Bermuda and about where it should have been on its journey to the canal. It is the most plausible and least controversial explanation for what became of the French submarine.

Despite the persistence of fanciful tales from New London, the collision had become largely accepted as the final chapter in Surcouf's story.

Until four years ago.

Capt. Frederick H. Hallett, a retired submariner, examined newly declassified documents and pieced together a more intriguing ending for Surcouf. His research was published in "The Submarine Review" in 2012.

First, he determined it was impossible for Surcouf to have reached the site of the Thompson Lykes collision by the time it happened.

Second, he said the British, eager to be rid of Surcouf but unable to retire it over Free French objections, sent it to Panama knowing it was likely to be attacked en route.

Third, he theorized that Surcouf's captain, realizing he was on a suicide mission, sailed to Vichy-controlled Martinique and defected, in keeping with the sympathies of much of his crew.

Finally, he asserts that Surcouf, departing Martinique for occupied France, was destroyed by depth charges from a U.S. air patrol in May 1942 after a U.S. destroyer was torpedoed nearby.

The truth remains a mystery till wreckage is found, but one place it's unlikely to turn up is Long Island Sound.

j.ruddy@theday.com