There was a moment of stunned silence as both Swifts realized that the telephone call had been faked!  Then Tom exclaimed: 

“Dad, this is serious!”

“Deadly serious, I agree,” his father replied.  “Are you calling from your lab?”

“Yes!”

“Stay there.  I’ll be right over,” the elder scientist said.

When Mr. Swift arrived, Tom related his conversation with the mysterious caller.  His father listened with worried eyes and a puzzled frown.

“It’s bad enough that an enemy was able to get the information,” Mr. Swift remarked.  “But, potentially at least, it’s even more dangerous that he was able to imitate my voice so well.  If he could fool you, Tom, he could fool anyone!”

“Are you thinking the same thing I am, Dad?”

“That it may have been some insider here at Enterprises?” When Tom nodded, his father gravely agreed.  “Yes, son, it does look that way.  To imitate my voice convincingly, it would almost certainly have to be someone who’s had close contact with us—­either at the plant or here in Shopton.”

The thought of a traitor at the experimental station was repugnant to the Swifts and to Bud as well.  Not only were all employees carefully screened, but there was a close, almost family relationship among those who took part in the exciting scientific ventures at Swift Enterprises.

Tom called Security and asked Harlan Ames to come over to the laboratory at once.  The security chief arrived within moments.  Quickly Tom filled him in on the details of the puzzling telephone call.

“Think back, skipper,” Ames urged.  “Was there anything at all you can remember about the voice that might give us a tip-off?  I mean, was it deep, or maybe a bit higher-pitched than you expected?  Or anything about the way the caller pronounced his words?”

Tom shook his head.  “Nothing.  That’s the trouble.  He spoke only a couple of sentences, but so far as I knew, it was my father calling!”

“Hmmm.”  Ames frowned.  “What about background noises?”

Tom thought hard.  “None.  If I had detected any special sounds during the call, I’m sure they would have stuck in my mind.”

Ames tried another tack.  He asked how many people had known about the expected arrival of the brain energy from space.  This was harder to answer, but as Tom and his father enumerated the persons, it did help to narrow the circle of suspects.

Besides the Swifts, Chow, Phyl, Ames, and George Dilling, there were three groups who had had access to the information.  One was the radio operators at the space-communications laboratory.  Another consisted of Arv Hanson and Hank Sterling and the workmen who had taken part in building the energy container.  The last group, which also included Hank and Arv, were the technicians who had actually gone to the hillside to await the visitor from Planet X.

Tom scowled.  “None of those people would pull such a trick, Harlan—­any more than the ones like you and Arv and Hank who are above suspicion.  Most of them could have easily obtained the news without going through such a rigmarole.”