"Impossible, Tom. The only people who enter the plant are known-" He stopped and stared at his son. "You mean we'd better look at the plant's personnel records?"

"Exactly, Dad."

Mr. Swift led his son to the security file in the main office.

"I don't know what we'll find here," the elder inventor said. "Every man is a top scientist. Three of them, the greatest in their respective fields. This card, for instance"-he held up a file microcard- "contains the record of one of the greatest physicists of our day, Robert Turnbull.

"The man has a number of research inventions to his credit, including the famous Turnbull Me-sonator. His entire family is scientifically gifted. He has an identical twin brother named Raymond, who is also an atomic physicist. He was a brilliant man but is now mentally ill."

"What happened, Dad?"

"The old story of a sensitive mind, overworked, I a.s.sume. My guess is that Raymond broke down from sheer mental exhaustion during his intensive research work."

Tom expressed sympathy and asked when the breakdown had occurred.

132 .

"Oh, about a year ago. Just before his brother Robert joined us here at the Citadel."

At that moment the blinker on the short-wave radio set lighted up. A message from Shopton was coming through.

"This is Phil Radnor!" barked the speaker. "We have a further report on Marco's activities while he was employed at Blackstone Mental Hospital. He seems to have been very friendly with one of the patients-a Raymond Turnbull- who has escaped!"

CHAPTER 16.

THE VULTURES RETURN.

AS RADNOR PAUSED after his startling announcement that the mentally ill scientist was missing, Tom exclaimed: "Rad, I want all the facts on the Turnbull escape! Get me the exact date."

Radnor reported that the escape had taken place exactly one month before.

"And what a month it's been!" Tom thought, remembering the attacks by crows on his jet plane and later the Sky Queen.

The loss of the relotrol had been followed by desperate attempts on the part of the Briggin gang to secure a robot. First the phony magician. Then the disguised Zoltan. Later Marco's strange behavior. A bomb at the plant and now a crow nearby.

"There must be some connection," Tom decided.

133.

134 .

After he finished his talk with Radnor, he turned to his father and expressed his suspicions.

"It seems incredible that a man of Raymond Turnbull's reputation would stoop to a.s.sociating with bank robbers," Mr. Swift said. "But a sick mind can lead to odd actions."

"Dad, how sure are you of Robert Turnbull's loyalty? Could he be mixed up in this thing too?"

Mr. Swift replied thoughtfully, "I can't believe he is guilty of any disloyalty to me. But I wonder how much he may know about his twin's recent activities."

"Don't you think that we should talk with Robert Turnbull at once?" Tom urged.

"Yes, son. I certainly don't want to cast suspicion on someone who may be innocent. But we must get to the bottom of this plot before our work and scientific progress is brought to a complete standstill."

They went immediately to the scientist's lab. Robert Turnbull was an erect, handsome man of fifty. With dark eyes set deep in his pale face, he looked quizzically at his visitors but greeted them affably. The Swifts told him of the report at once.

Robert Turnbull listened in silence for several minutes, then suddenly he flung his lab ap.r.o.n angrily to the floor.

"Mr. Swift, I'm shocked at your insinuations!" he snapped.

"I believe," said Mr. Swift with dignity, "that if THE VULTURES RETURN 135.

you were in my place you would make an investigation too."

Turnbull calmed down a bit. Retrieving his ap.r.o.n, he tilted back on the lab stool and regarded Mr. Swift and his son curiously. "I suffered months of abuse and worry because of my twin brother's strange actions before he entered Blackstone. I thought all that was past. Now you come to me with a story of an escape I know nothing about.

"Then you dare to accuse a sick man who has devoted his life to the betterment of humanity of collaborating with criminal elements and conspiring to commit sabotage. No Turnbull would ever be connected with such underhanded treachery!"

"You say you know nothing about your brother's escape?" Tom queried.

"I am shocked by the news and angry that my family has not informed me of it."

"Perhaps they do not know about his escape," Tom ventured.

Robert Turnbull got off the stool so violently that it toppled over. "I must leave at once and contact my family to learn if they have any information about Raymond. Perhaps he has sought shelter with them. Poor Raymond!"

The scientist gathered his work together, scooped up a sheaf of papers, and hurried out of the room. Feeling justified about the questioning but sorry that it had upset Robert Turnbull, the Swifts returned to their own office.

136 .

No sooner had they entered than the blinker monitor again registered a call from Shopton.

Tom gave a mock groan and said, "I wonder if this is more trouble?"

From the other end of the line, Radnor's voice was hollow. "Hate to bother you again, but something has come up."

The young inventor steadied himself. "What's wrong, Rad?" he asked tersely.

"Marco is gone!" Rad reported.

Tom winced. Their key contact had vanished!

Radnor said Marco had melted into a downtown crowd, eluding the shadowing detective. The next evening he had not appeared for work. That could only mean the watchman had found out that he was under suspicion.

It also implied that he probably had learned about the fake messages planted on the tape. He might have gathered the real facts through some other source.

Tom informed Radnor about the conference with Robert Turnbull, then signed off.

"I'm afraid we're in for more trouble," Tom said, and told his father about Radnor's report.

"The Citadel and everyone in it is probably a target now," Tom continued.

He did not add that he feared for the safety of Sandy and Phyl too. He wished, now, that they had not accompanied him on what was proving to be a hazardous venture.

Meanwhile, the Skeeter was riding the canyon up- THE VULTURES RETURN 137.

drafts. Under Bud Barclay's skillful handling, the helicopter had covered many miles of scenic eroded rock, hovered directly in front of grotesque pink cliffs, and whirled around jagged, fiery-orange stone formations. He windmilled the craft under a natural limestone arch while Sandy snapped pictures and Phyl drew quick sketches.

Later, they pa.s.sed over Indian pueblo dwellings. The adobe skysc.r.a.pers, heaped atop one another, rose like rock-tiered tables out of the loam. Through binoculars Sandy could plainly see the bright-colored blankets that the Indians used for doors.

After pa.s.sing over a stretch of rolling land dotted with sagebrush, they saw Purple Mesa. It stood like a solid fortress in the lighter-colored landscape.

"We're almost there!" called Sandy excitedly, as the huge ma.s.s loomed before them.

"It's still a number of miles off," Bud observed. "Distances are deceiving out here."

The mesa was indeed several minutes' flying time away. Alone and brooding, it seemed to bear down upon them as they approached.

"Why, it isn't purple at all!" exclaimed Phyl. "It seems to be rust-colored."

"Wait until sunset," Bud remarked.

"We won't be here then," said Phyl, a note of disappointment in her voice.

"Tom made us promise to be back by suppertime."

"I'll take the Skeeter up. We'll hover over the top and look for a landing place," said Bud.

138 .

The helicopter rose alongside the sheer wall of Purple Mesa.

"It is steep," gasped Sandy, "and craggy. No wonder n.o.body's been able to climb it to look for the treasure!"

The cliff's edges had been filed into sharp and fantastic shapes by the countless desert sandstorms. Bud carefully spiraled the Skeeter in for a landing on the flat top of the mesa.

"Oh, look!" cried Phyl. "Here comes a family of vultures. They must nest on the mesa."

As Bud held the ship steady, he glanced over his shoulder. The birds were not vultures but metal-feathered jets. They were the death-dealing crows, capable of seizing complete control of the helicopter and dashing it to pieces against the rocks.

Suddenly the ship was buffeted around.

"What's the matter with the Skeeter?" Sandy cried.

"The birds!" yelled Bud. He kicked desperately at the control pedals, but it was no use. The stabilizer rotor was out of control and the cabin began to spin.

"Hang on, we're going to crash!" Bud shouted in warning.

The helicopter dropped, hit the edge of the mesa, and plummeted down the side!

CHAPTER 17.

MAROONED ON THE MESA.

THE SKEETER hung on a crag at the edge of the precipice.

"Kick the window!" Bud yelled.

Sandy's foot flew against the large pane of safety gla.s.s. The pane popped out and Sandy tumbled out onto the top of the mesa. Phyl scrambled after her just as the helicopter started to roll down the steep cliff.

The girls watched in horror as the craft, with Bud still trapped inside, grated noisily down the incline. A rotor blade snapped off and went spinning away. A moment later a formation of up-jutting rocks caught the Skeeter like a giant outstretched hand.

The girls stared blankly at the wreckage, hoping against hope that Bud was still alive. As they waited, 139.

A formation of up-jutting rocks caught the MAROONED ON THE MESA.

141.

frantic because they could not help, the seconds seemed like centuries.

Suddenly Sandy grabbed Phyl's arm. She had heard a faraway creaking sound. Slowly the twisted door of the helicopter was being forced open. Bud staggered out, seemingly uninjured. The girls called down to him.

"I feel like a one-wheeled tricycle," Bud replied. His voice was barely audible to the girls. "Pretty banged up but all in one piece."

Sandy and Phyl sighed in relief but their elation was short-lived. Bud was still trapped! Hanging precariously, halfway down the cliff, he could neither climb down nor locate any footholds for an ascent.

Bud, realizing the near futility of his situation, knew he must not become panicky.

Settling back against the helicopter, he surveyed the scene. A descent was out of the question. The cliff walls rose in a sheer line from the desert floor. One slip and he would be battered against broken boulders that fanned out at the base.

His only chance was to risk a climb. He would have to do it without the a.s.sistance of caulked climb-

Skeeter like a giant outstretched hand 142 .

ing shoes or a pickax. But one essential he could not do without was a rope.