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About three hours before Jennifer Schipsi's body was found in a burned cottage on Addison Avenue in Palo Alto, her cell phone appeared to be traveling with her boyfriend Bulos Zumot's phone, a cell-phone expert testified Friday.

Jim Cook, who used reports from AT&T to analyze call data on both iPhones, testified Friday that the phones appeared to be using the same cell tower on at least three occasions on the afternoon of Oct. 15. He cited specific cell towers that he said both phones had used that day, including ones in San Jose, near Sunnyvale and in Palo Alto.

Cook, a former AT&T field representative who now trains law-enforcement officials on analyzing wireless devices, presented the jury in Zumot's murder-arson trial Friday with detailed spreadsheets listing all calls and text messages made and received by Zumot and Schipsi, including ones they exchanged between themselves.

He also produced a series of maps illustrating the coverage areas of the towers the two cell phones used between the evening of Oct. 14 and the evening of Oct. 15.

Cook's testimony was instantly disputed by Zumot's attorney Mark Geragos, who produced his own AT&T records and argued that the data Cook relied on doesn't actually exist. Geragos showed the jury the AT&T report listing the call data from the two phones. For the three phone calls in which the two phones appeared to be traveling together, the fields that normally display data for cell towers were blank.

Geragos blasted Cook's findings and told him to produce the AT&T report on which he based his conclusions. When Cook couldn't produce that report, Geragos pointed out repeatedly that the numbers Cook was showing the jurors was an Excel spreadsheet that Cook himself had put together -- not raw data from the company.

"There isn't a single record from AT&T that has this cell-tower data," Geragos said during his cross-examination.

Cook said that while the data was missing in this report, he requested more information from AT&T later. He said all the data he used in his analysis came from AT&T. He also said that he had not read any police reports relating to this case.

Cell-phone data is one of the key components of the prosecution's case against Zumot, a Palo Alto resident who was arrested on Oct. 19 and charged with killing Schipsi and then burning down their shared cottage to cover up the crime. During his opening argument last week, prosecutor Charles Gillingham showed the jury maps illustrating cell-tower locations as part of his argument that Zumot had Schipsi's phone in his possession after killing her.

Earlier in the week, the jury heard from Jaber Al Suwaidi, a friend of both Zumot and Schipsi, who said he received a text message from Schipsi's phone on the afternoon of Oct. 15. He said he believed, based on the writing, that the message was actually written by Zumot.

Cook began his testimony on Thursday with a detailed account of calls and messages sent by Zumot's phone on Oct, 14, 2009, and Oct. 15, 2009. He did the same thing Friday for Schipsi's phone.

Gillingham went over the list of calls and messages with Cook, who identified the cell towers and the sectors on the towers that were used for these calls and messages. At one point, just as the jury was preparing for its morning recess, Zumot's brother, David, who was sitting in the audience, made a remark. When Gillingham began to approach him to ask him about what he said, David Zumot said he was talking to himself.

Once outside the courtroom, David Zumot said he said to himself, "Things aren't working out," in reference to the prosecution's case.

Geragos will continue his cross-examination of Cook when the trial resumes on Jan. 24.

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■ Friend doubted Schipsi wrote him a message

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■ Zumot's friend recalls night before fatal fire

■ Police handling of Schipsi murder challenged

■ Zumot-trial attorneys clash over arson evidence

■ Cell phone takes center stage at Zumot trial

■ Boyfriend arrested in Palo Alto death of Jennifer Marie Schipsi