By Andrew Grossman 

NEW YORK--Federal Bureau of Investigation Director James Comey on Wednesday said the U.S. is confident about North Korea's involvement in the December threats against Sony Pictures because the people involved at times slipped up and didn't properly use tactics designed to obscure the source of the messages.

When that happened, investigators were able to see clearly that they came from Internet addresses used solely by North Korea, Mr. Comey said.

"There is not much in this life that I have high confidence about," Mr. Comey said. "I have very high confidence about this attribution."

Mr. Comey also highlighted other evidence, such as analysis by FBI personnel that matched patterns of writing and other signatures to those found in other attacks launched by North Korea.

"They say: 'easy for us,' " Mr. Comey said of the bureau's analysts. "It's the same actors."

The Wall Street Journal has previously reported investigators had discovered an instance when malicious software tried to contact an address in North Korea.

Mr. Comey said the bureau is still trying to figure out how the hackers got into Sony's systems, and noted there had been attempts at "spearphishing" launched against the company as late as September. Spearphishing refers to efforts by hackers to gain access to systems by sending targeted messages to specific people designed to get them to do something that would compromise security, such as share their credentials or click a link that installs malicious software.

Mr. Comey said the U.S. has more evidence that North Korea was behind the attack that it can't release publicly. He said those who have questioned the conclusion North Korea was involved "don't have the facts that I have, they don't see what I see."

Write to Andrew Grossman at andrew.grossman@wsj.com

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