I was searching through newspapers.com for casualty announcements in June and July 1861, and came across a previously unknown skirmish near Alexandria, Virginia that took place on or around June 30th. At first I thought it was referring to the Engagement at Arlington Mills, but that happened almost a month earlier. I haven’t seen this specific incident mentioned in the usual lists. Here is the newspaper article, in the Richmond Dispatch, July 4, 1861, page 3:

THE SKIRMISH NEAR ALEXANDRIA
The following account of the skirmish in which Sergeant Hanes, of the Governor’s Guard, lost his life, was telegraphed North from Alexandria, on the 30th of June:
This morning, at daybreak, fourteen Confederate scouts attacked three picket guards of the 14th Pennsylvania Regiment, belonging to Company S, Capt. Amer, stationed on Shouter’s Hill, Virginia, four miles from Alexandria, wounding Lewellan Rumor, of Blue Bell, and killing Thomas Murray, of Norristown; the pickets returned the fire, killing two Confederates, and wounding a third. One of the slain was a sergeant of the Letcher Guard. The enemy beat a hasty retreat.
The firing having been heard by the Federal troops, a detachment of the Zouaves and another of the Fourth Pennsylvania Regiment reinforced the pickets and followed in the trail of the enemy for some distance, finding four rifles and three revolvers, which the latter threw away in their hasty flight. One of the revolvers was a very valuable article, marked with the name of John Johnson, a farmer living in that vicinity, who is a noted Secessionist.
The Pennsylvanians behaved with great spirit and with the coolness of veterans, boldly holding their positions, though wounded, in the hope of being reinforced. The body of Murray was brought to Washington this afternoon, and will be forwarded to Norristown. The Federal troops express themselves sadly disappointed at not taking or killing Johnson, as he has been a very troublesome man to the Unionists by reason of his thorough knowledge of the localities thereabout.
AFFAIRS IN ALEXANDRIA
Alexandria, July 1 — The Confederate killed by the Pennsylvania 4th picket, named Henry C. Hanes, is a well-known citizen of Richmond, and orderly sergeant of the Letcher Guards. He was buried today by the citizens there, his body having been transferred to their charge at their request. The two other Secessionists who were wounded have since died.
[It is positively known that Mr. Hanes was the only one of our men injured in the skirmish. However, we never expect truth from the Yankees.]
So what can we dig up? At first glance, I found the “Letcher Guard” was Company A of the 9th Battalion, Virginia Infantry (Hansbrough’s). But this unit was raised in what was the Taylor County, Virginia (today, WV). This doesn’t seem right, as other sources say Sgt. Henry C. Hanes was a resident of Henrico County, Virginia. Also, the 9th Battalion was stationed in northwestern Virginia, nowhere near Alexandria.
There was a Richmond unit called the “Governor’s Mounted Guard,” that could reasonably be referred to as “Letcher’s Guard,” after Virginia Governor John Letcher. This cavalry company was commanded by Capt. John Grattan Cabell, member of a prominent Richmond family. The Governor’s Mounted Guard was stationed in Fairfax County keeping watch on Union activities in the area, so this is most likely the unit referred to in the article.
Clearly, much more research is needed to establish exactly what units were involved in this skirmish, and who became casualties. Stay tuned for updates.