"Stop Stonewalling."

That's the warning from Arizona lawmakers hoping to find out what really happened earlier this month when four Tennessee National Guardsmen reportedly retreated when confronted by armed illegal immigrants along the border south of Tucson.

So far, Guard and U.S. Border Patrol officials have refused to disclose exactly what happened Jan. 3 when gunmen assaulted a Guard lookout post near Sasabe, Ariz. They declined requests from FOX News for copies of incident reports and transcripts of interviews with the men involved.

"Unfortunately, we do not have a report to provide," said Michael Friel, the U.S. Customs and Border Protection spokesman.

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On Monday, Maj. Gen. David Rataczak was to appear before the Arizona House Homeland Security Committee to testify about the encounter.

"What are they here for if they are going to retreat from people with automatic weapons?" asked Committee Chairman Warde Nichols, who said the incident may send the message that the National Guard will retreat if faced with armed individuals. "It is not in the best interest of Arizona or U.S. border security," he added.

Rep. Steve Gallardo, a Democrat on the committee, said he believed immigration hard-liners would use Rataczak's appearance to push their agenda.

"They are going to try and embarrass him. They are going to fail," Gallardo said.

The incident happened at night, about a quarter mile north of the U.S. border with Mexico. A spokesman for the Arizona National Guard said an undetermined number of armed men approached an E.I.T., or Entry Identification Team, from Tennessee. Dozens of these mobile lookout posts are set up along the border, several are near Sasabe, a popular drug corridor. An E.I.T. is typically manned by four Guard soldiers equipped with radios, night vision and other surveillance gear.

Under existing rules of force signed by the Department of Defense and border state governors, soldiers are not supposed to stop, arrest, or shoot armed illegal immigrants. They are instructed only to look, listen and report their location to the Border Patrol.

"We don't apprehend," said Maj. Paul Aguirre, a spokesman for the Arizona National Guard. "We don't detain. We don't transport."

For that reason, critics say, it is inaccurate to say the National Guard is protecting the border.

While Guard spokesman Paul Aguirre called the encounter a "non-incident," U.S. Border Patrol sources in Tucson familiar with the investigation say something entirely different. They describe a tense, armed confrontation, with both sides lifting their assault rifles to shoulder height.

The sources say 12 men assaulted the Guard position, dressed in black tactical vests and khaki military style fatigues. The unit split into two groups as it approached, with eight men in front and two men flanking the Guardsmen on each side. One of the gunmen came within 35 feet of the observation site, according to investigators' summaries. Surrounded, outmanned and outgunned, the four Guardsmen made a "tactical retreat" to their Humvee and called the Border Patrol, the sources said.

The Border Patrol tracked the armed men back to the border but could not locate them. No shots were fired.

Guard spokesman Aguirre objected to characterizations of the withdrawal as a retreat, saying the soldiers did not run from their post and were not overrun.

The troops monitored the situation, never lost contact with the gunmen and moved to another site to avoid an engagement, Aguirre said.

Democratic Gov. Janet Napolitano, National Guard officials and some state lawmakers defended the decision to call in the Border Patrol. The governor's office has said the rules allow Guard members to use force when they believe they face an imminent threat and all other means are exhausted.

"I don't think that it's up to the committee to negotiate the rules of engagement," Napolitano said. "Those rules of engagement were negotiated with the National Guard at the federal level."

Border agents interviewed over the weekend believe the group was military trained, and were likely ex-Mexican special forces working for the drug cartels or a rival cartel 'rip-off' squad that steals drug shipments once they've crossed the border.

Initial reports suggested the Guardsmen were unarmed. However, Border Patrol spokesman Gustavo Soto said the teams "had rifles and ammunition from Day One."

That is true for the E.I.T. teams, but local agents say most Guardsmen involved with Operation Jump Start — those resurfacing roads and building fences — are not armed because officials "don't want an incident."

"The stories we've gotten from the National Guard, quite frankly, have changed," said lawmaker Nichols. "What happened that day? Is this isolated incident? Does it happen often armed men come across border in Kevlar vests moving in tactical formation and come within 30 feet of a National Guard post? We need to know."

The four Tennessee Guardsmen involved in the "tactical retreat," or redeployment, were to be honored in Tucson Monday in a closed ceremony. An Arizona Guard spokeswoman refused to identify the medal or ribbon or commendation being given out, and said the press was not invited.

The troops were among the 6,400 National Guard members sent to the four southern border states to support immigration agents, and leave the agents with more time to catch illegal immigrants.

The support duties include monitoring border points, assisting with cargo inspection and operating surveillance cameras.

FOX News' William LaJeunesse and The Associated Press contributed to this report.