"Yes, I crossed the line," Arneson said. "I've taken responsibility for that, and I will continue to."
Arneson is accused of taking bribes in excess of $180,000 as part of a scheme orchestrated by private eye Anthony Pellicano that used clandestine tactics to dig up dirt to aid clients in legal and other disputes.
All five defendants have pleaded not guilty.
Arneson testified that the money he received from Pellicano was for off-duty security work, including escorting stars to award shows and conducting surveillance.
"The payments you received were never in connection" with running names in databases, his defense attorney, Chad Hummel, said during questioning.
"That's correct," Arneson replied.
Arneson said Pellicano didn't tell him why he wanted the names run through the databases, or how the information would be used.
In some cases, Pellicano had information before the data was accessed, Arneson said.
"I was affirming information he already had," Arneson testified. "I was eliminating his legwork."
Arneson said he also ran names for other people as personal favors.
Arneson testified that one request came from FBI Special Agent Stanley Ornellas, who wanted to find out information about his neighbor. Ornellas went on to become the lead investigator in the Pellicano case. He is now retired.
Ornellas did not immediately return a request for comment made Friday through FBI spokeswoman Laura Eimiller.
Earlier in the day, an FBI agent testified that he was unable to decrypt some audio files seized during raids on the offices of Pellicano that began in November 2002.
Agent Donald Schmidt was the only witness called by Pellicano, who is acting as his own attorney. Pellicano reserved the right to testify on his own behalf later in the trial.
Lawyers for the other defendants also had cases to present.
Pellicano questioned Schmidt, an information technology specialist, for about an hour. The agent testified that he was instructed to look for calls that had been wiretapped among the recordings.
He said he was unable to listen to some of the encrypted calls.
"Did you find any evidence of wiretaps?" Pellicano asked.
"I found what sounded like telephone recordings," Schmidt replied.
During the trial, prosecutors played at least six recordings of calls between Pellicano and clients but only one tape they characterized as a wiretap.